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.

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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.

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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.

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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.

test/functional/terminal/buffer_spec.lua

@@ -446,8 +446,10 @@ end)
 describe('terminal input', function()
   it('sends various special keys with modifiers', function()
     clear()
-    local screen = thelpers.screen_setup(0,
-      string.format([=[["%s", "-u", "NONE", "-i", "NONE", "--cmd", "startinsert"]]=], nvim_prog))
+    local screen = thelpers.screen_setup(
+      0,
+      string.format([=[["%s", "-u", "NONE", "-i", "NONE", "--cmd", 'colorscheme vim', "--cmd", "startinsert"]]=], nvim_prog)
+    )
     screen:expect{grid=[[
       {1: }                                                 |
       {4:~                                                 }|