Problem:
The magic globals `it`, `describe`, etc., are more trouble than they are
worth.

- Hooking into `after_each` requires `getfenv()` hacks.
- They confuse luals/emmylua, because the top-level `.luarc.json` isn't
  merged with `test/.luarc.json` (apparently a luals limitation?)
- They totally defeat discoverability because the user just has to
  "know" about the various magic symbols.

So they harm DX, which means they serve no purpose at all.

Solution:
- Expose the test API from `testutil`, so tests can call `t.it()`,
  `t.describe()`, etc., in the conventional way.
- Drop `getfenv()` hacks.
- Drop the `setfenv()` injection in `load_chunk`.
- Drop `test/_meta.lua`.

Problem:  close_buffer() callers incorrectly handle b_nwindows,
          especially after nasty autocmds, allowing it to go
          out-of-sync.  May lead to buffers that can't be unloaded, or
          buffers that are prematurely freed whilst displayed.
Solution: Modify close_buffer() and review its callers; let them
          decrement b_nwindows if it didn't unload the buffer.  Remove
          some now unneeded workarounds like 8.2.2354, 9.1.0143,
          9.1.0764, which didn't always work (Sean Dewar)

(endless yapping omitted)

related: vim/vim#19728

https://github.com/vim/vim/commit/bf21df1c7bc772e3a29961c961d0821584d50ee0

b_nwindows = 0 change for free_all_mem() was already ported.

Originally Nvim returned true when b_nwindows was decremented before the end was
reached (to better indicate the decrement). That's not needed anymore, so just
return true only at the end, like Vim. (retval isn't used anywhere now anyways)

Set textlock for dict watchers at the end of close_buffer() to prevent them from
switching windows, as that can leave a window with a NULL buffer. (possible
before this PR, but the new assert catches it; added a test)

Despite textlock, things still aren't ideal, as watchers may observe the buffer
as unloaded and hidden (b_nwindows was decremented), yet still in a window...
Likewise, for Nvim, wipe_qf_buffer()'s comment may not be entirely accurate;
autocmds are blocked, but on_detach callbacks (textlocked) and dict watchers may
still run. Might be problematic, but those aren't new issues.

Co-authored-by: Sean Dewar <6256228+seandewar@users.noreply.github.com>

Work on https://github.com/neovim/neovim/issues/27004.

Problem:
Not all Lua code is checked by stylua. Automating code-style is an
important mechanism for reducing time spent on accidental
(non-essential) complexity.

Solution:
- Enable lintlua for `test/unit/` directory.
- TODO: only `test/functional/` remains unchecked.

previous: 45fe4d11add933df76a2ea4bf52ce8904f4a778b
previous: 517f0cc634b985057da5b95cf4ad659ee456a77e

Problem:    Unable to customize the column next to a window ('gutter').
Solution:   Add 'statuscolumn' option that follows the 'statusline' syntax,
	    allowing to customize the status column. Also supporting the %@
	    click execute function label. Adds new items @C and @s which
	    will print the fold and sign columns. Line numbers and signs
	    can be clicked, highlighted, aligned, transformed, margined etc.

Problem:    Handling 'statusline' errors is spread out.
Solution:   Pass the option name to the lower levels so the option can be
            reset there when an error is encountered. (Luuk van Baal,
            closes vim/vim#11467)

https://github.com/vim/vim/commit/7b224fdf4a29f115567d4fc8629c1cef92d8444a

problem: code for drawing statusline is arbitrarily spreadout between drawscreen.c, screen.c and buffer.c
solution: move it to a new file statusline.c

- rename archaic internal name "status match" to public name "wildmenu"
- showruler() does not show the ruler. it show anything which displays
  info about the cursor. Rename it accordingy.