fix(colorscheme): adjust StatusLine[NC] for diagnostics #36719

Problem: It is desirable for the default statusline to contain colored
  diagnostics information. However, current `StatusLine` group is
  purposefully defined as almost inverted `Normal` to "make current
  window obvious". This makes diagnostic information from
  `vim.diagnostic.status()` barely visible: it uses established
  `DiagnosticSignXxx` groups which have colored foreground with
  lightness close to `StatusLine` background.

  Also the `StatusLineNC` group is fairly different from `Normal` in
  order to both "makes window separators clear" and "be different from
  `CursorLine`". But not as mush different as `StatusLine` because
  "`StatusLine` and `StatusLineNC` should be clearly different".

Solution: Make both `StatusLine` and `StatusLineNC` be slightly closer
  in lightness to `Normal`. This makes `StatusLine` and `StatusLineNC`
  groups satisfy their conditions in the following way:
  - `vim.diagnostic.count()` is readable on `StatusLine` - yes.
  - `vim.diagnostic.count()` is readable on `StatusLineNC` - yes.
  - `StatusLine` makes current window obvious - I'd say yes.
  - `StatusLine` and `StatusLineNC` are clearly different - it depends
    on the eyes and monitor. The current is clearly better, but the new
    ones I'd say are still visibly different.
  - `StatusLineNC` makes window separators clear - I'd say yes, but
    depends on the eyes and monitor.
  - `StatuslineNC` is different from `CursorLine` - NO, they are same.

  Another approach to solve this would be to introduce dedicated
  `DiagnosticStatuslineXxx` groups to use in `vim.diagnostics.status()`.
  They can be defined using foreground colors from the same lightness as
  `Normal`. This would make them readable in `StatusLine`. But not
  `StatusLineNC`, though.

test/functional/ui/embed_spec.lua

@@ -30,8 +30,8 @@ local function test_embed(ext_linegrid)
       [100] = { foreground = Screen.colors.NvimDarkCyan },
       [101] = { foreground = Screen.colors.NvimDarkRed },
       [102] = {
-        background = Screen.colors.NvimDarkGrey3,
-        foreground = Screen.colors.NvimLightGrey3,
+        background = Screen.colors.NvimLightGrey4,
+        foreground = Screen.colors.NvimDarkGrey2,
       },
     }
   end