feat(highlight): tweak default color scheme

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.

test/functional/core/main_spec.lua

@@ -72,15 +72,12 @@ describe('command-line option', function()
         env = { VIMRUNTIME = os.getenv('VIMRUNTIME') }
       })
       screen:expect([[
-        {1:^                                        }|
-        {2:~                                       }|*4
-        {3:[No Name]             0,0-1          All}|
-        {1:                                        }|
-                                                |
+        ^                                        |
+        ~                                       |*4
+        {1:[No Name]             0,0-1          All}|
+                                                |*2
       ]], {
-        [1] = {bg_indexed = true, foreground = Screen.colors.Grey91, background = tonumber('0x161616'), fg_indexed = true},
-        [2] = {bg_indexed = true, foreground = Screen.colors.Gray30, background = tonumber('0x161616'), fg_indexed = true},
-        [3] = {bg_indexed = true, foreground = tonumber('0xd2d2d2'), background = Screen.colors.Black , fg_indexed = true},
+        [1] = {reverse = true};
       })
       feed('i:cq<CR>')
       screen:expect([[