feat(highlight): update 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.

1 files changed, 21 insertions, 15 deletions

diff --git a/test/functional/ui/embed_spec.lua b/test/functional/ui/embed_spec.lua
index 9729f65355..3bc3af9853 100644
--- a/test/functional/ui/embed_spec.lua
+++ b/test/functional/ui/embed_spec.lua
@@ -26,6 +26,12 @@ local function test_embed(ext_linegrid)
       [3] = {bold = true, foreground = Screen.colors.Blue1},
       [4] = {bold = true, foreground = Screen.colors.Green},
       [5] = {bold = true, reverse = true},
+      [6] = {foreground = Screen.colors.NvimDarkGrey3, background = Screen.colors.NvimLightGrey1};
+      [7] = {foreground = Screen.colors.NvimDarkRed};
+      [8] = {foreground = Screen.colors.NvimDarkCyan};
+      [9] = {foreground = Screen.colors.NvimLightGrey3, background = Screen.colors.NvimDarkGrey1};
+      [10] = {foreground = Screen.colors.NvimLightRed};
+      [11] = {foreground = Screen.colors.NvimLightCyan};
     })
   end
 
@@ -36,10 +42,10 @@ local function test_embed(ext_linegrid)
                                                                   |
                                                                   |
                                                                   |
-                                                                  |
-      Error detected while processing pre-vimrc command line:     |
-      E121: Undefined variable: invalid                           |
-      Press ENTER or type command to continue^                     |
+      {6:                                                            }|
+      {7:Error detected while processing pre-vimrc command line:}     |
+      {7:E121: Undefined variable: invalid}                           |
+      {8:Press ENTER or type command to continue}^                     |
     ]])
 
     feed('<cr>')
@@ -64,11 +70,11 @@ local function test_embed(ext_linegrid)
                                                                   |
                                                                   |
                                                                   |
-      {5:                                                            }|
-      Error detected while processing pre-vimrc command line:     |
-      foo                                                         |
-      {1:bar}                                                         |
-      {4:Press ENTER or type command to continue}^                     |
+      {9:                                                            }|
+      {7:Error detected while processing pre-vimrc command line:}     |
+      {7:foo}                                                         |
+      {10:bar}                                                         |
+      {11:Press ENTER or type command to continue}^                     |
     ]])
   end)
 
@@ -78,11 +84,11 @@ local function test_embed(ext_linegrid)
                                                                   |
                                                                   |
                                                                   |
-                                                                  |
-      Error detected while processing pre-vimrc command line:     |
-      foo                                                         |
-      bar                                                         |
-      Press ENTER or type command to continue^                     |
+      {6:                                                            }|
+      {7:Error detected while processing pre-vimrc command line:}     |
+      {7:foo}                                                         |
+      {7:bar}                                                         |
+      {8:Press ENTER or type command to continue}^                     |
     ]], condition=function()
       eq(Screen.colors.Green, screen.default_colors.rgb_bg)
     end}
@@ -144,7 +150,7 @@ describe('--embed --listen UI', function()
     helpers.skip(helpers.is_os('win'))
     clear()
     local child_server = assert(helpers.new_pipename())
-    funcs.jobstart({nvim_prog, '--embed', '--listen', child_server, '--clean'})
+    funcs.jobstart({nvim_prog, '--embed', '--listen', child_server, '--clean', '--cmd', 'colorscheme vim'})
     retry(nil, nil, function() neq(nil, uv.fs_stat(child_server)) end)
 
     local child_session = helpers.connect(child_server)