Problem:  Visual highlight hard to read with 'termguicolors'
          (Maxim Kim)
Solution: Set Visual GUI foreground to black (with background=light)
          and lightgrey (with background=dark)
          (Maxim Kim)

fixes: vim/vim#14024
closes: vim/vim#14025

https://github.com/vim/vim/commit/34e4a05d02a016fe230495be8f6c60ddd56f9567

Co-authored-by: Maxim Kim <habamax@gmail.com>

Problem:  Visual highlighting can still be improved
Solution: Update Visual highlighting for 8 color terminals,
          use uniform grey highlighting for dark and light bg
          (Maxim Kim)

Update terminal Visual

1. Use `ctermbg=Grey ctermfg=Black` for both dark and light

This uniforms Visual highlighting between default dark and light colors
And should work for vim usually detecting light background for terminals
with black/dark background colors.

Previously used `ctermfg=White` leaks `cterm=bold` if available colors
are less than 16.

2. Use `term=reverse cterm=reverse ctermbg=NONE ctermfg=NONE`
   for terminals reporting less than 8 colors available

If the terminal has less than 8 colors, grey just doesn't work right

closes: vim/vim#13940

https://github.com/vim/vim/commit/59bafc8171b08cf326ed40ccb4ee917f9643290e

Co-authored-by: Maxim Kim <habamax@gmail.com>

Problem:  UX of visual highlighting can be improved
Solution: Improve readibility of visual highlighting,
          by setting better foreground and background
          colors

The default visual highlighting currently is nice in that it overlays
the actual syntax highlighting by using a separate distinct background
color.

However, this can cause hard to read text, because the contrast
between the actual syntax element and the background color is way too
low. That is an issue, that has been bothering colorschemes authors for
quite some time so much, that they are defining the Visual highlighting
group to use a separate foreground and background color, so that the
syntax highlighting vanishes, but the text remains readable (ref:
vim/colorschemesvim/vim#250)

So this is an attempt to perform the same fix for the default Visual
highlighting and just use a default foreground and background color
instead of using reverse.

I also removed the hard-coded changes to the Visual highlighting in
init_highlight. It's not quite clear to me, why those were there and not
added directly to the highlighting_init_<dark|light> struct.

closes: vim/vim#13663
related: vim/colorschemes#250

https://github.com/vim/vim/commit/e6d8b4662ddf9356da53f56e363b67b524fd8825

Co-authored-by: Christian Brabandt <cb@256bit.org>

runtime(colorschemes): Add initial support for Added/Removed/Changed highlight groups (vim/vim#13830)

For some of the colorschemes where diffAdded and diffRemoved were explicitly set up.

https://github.com/vim/vim/commit/5f4cc8ea65e80efa35f84fe9a9985a42c3c96bae

Co-authored-by: Maxim Kim <habamax@gmail.com>

Problem:
Bundled color schemes use `:hi clear` and only define Vim's highlight
groups. This results into Nvim-specific highlight groups using
definitions from Nvim's default color scheme, which are not always
linked to a Vim's highlight group.

Solution:
Restore links to Vim's highlight groups which were present before Nvim's
default color scheme update.

Problem: Bundled 'vim' color scheme is written in Vimscript which
  implicitly assumes that the file is ported from Vim.
  This is not the case, at it is currently the Neovim's way of providing
  backward compatibility for color schemes.

Solution: Rewrite it in Lua to indicate that this runtime file comes
  from Neovim.

runtime(colorschemes): Update colorschemes (vim/vim#13691)

Minor changes:

- Regenerated using colortemplate 2.2.3
- Removed reversed gui-cursor for some of the colorschemes (retrobox, wildcharm, lunaperche)
- Change MatchParen for some of colorschemes.

https://github.com/vim/vim/commit/0378f07fbaee9fd8773f3cf8cef146606dd7bf72

Co-authored-by: Maxim Kim <habamax@gmail.com>

Problem: Updating default color scheme produced some feedback.

Solution: Address the feedback.

Outline of the changes:

- Colors `Grey1` and `Grey2` are made a little bit more extreme (dark -
  darker, light - lighter) to increase overall contrast.

- `gui` colors are treated as base with `cterm` colors falling back to
  using 0-15 colors which come from terminal emulator.

- Update highlight group definition to not include attribute definition
  if it is intended to staty uncolored.

- Tweak some specific highlight groups.

- Add a list of Neovim specific highlight groups which are now defined
  differently in a breaking way.

- Minor tweaks in several other places related to default color scheme.

Problem: Default color scheme is suboptimal.

Solution: Start using new color scheme. Introduce new `vim` color scheme
for opt-in backward compatibility.

------
Main design ideas
- Be "Neovim branded".
- Be minimal for 256 colors with a bit more shades for true colors.
- Be accessible through high enough contrast ratios.
- Be suitable for dark and light backgrounds via exchange of dark and
  light palettes.

------
Palettes

- Have dark and light variants. Implemented through exporeted
  `NvimDark*` and `NvimLight*` hex colors.

- Palettes have 4 shades of grey for UI elements and 6 colors (red,
  yellow, green, cyan, blue, magenta).

- Actual values are computed procedurally in Oklch color space based on
  a handful of hyperparameters.

- Each color has a 256 colors variant with perceptually closest color.

------
Highlight groups

Use:

- Grey shades for general UI according to their design.

- Bold text for keywords (`Statement` highlight group). This is an
  important choice to increase accessibility for people with color
  deficiencies, as it doesn't rely on actual color.

- Green for strings, `DiffAdd` (as background), `DiagnosticOk`, and some
  minor text UI elements.

- Cyan as main syntax color, i.e. for function usage (`Function`
  highlight group), `DiffText`, `DiagnosticInfo`, and some minor text UI
  elements.

- Red to generally mean high user attention, i.e. errors; in particular
  for `ErrorMsg`, `DiffDelete`, `DiagnosticError`.

- Yellow very sparingly only with true colors to mean mild user
  attention, i.e. warnings. That is, `DiagnosticWarn` and `WarningMsg`.

- Blue very sparingly only with true colors as `DiagnosticHint` and some
  additional important syntax group (like `Identifier`).

- Magenta very carefully (if at all).

------
Notes

- To make tests work without relatively larege updates, each one is
  prepended with an equivalent of the call `:colorscheme vim`.

  Plus some tests which spawn new Neovim instances also now use 'vim'
  color scheme.

  In some cases tests are updated to fit new default color scheme.