Syntax highlighting in diff mode

[Niverton]

Is your feature request related to a problem? Please describe.
When using diff mode, the text is black on a bright background (in dark mode). This is not nice on the eye, and makes large changes in a code base (big DiffAdd blocks especially) hard to read due to the lack of syntax highlighting. See #105 for previous discussions on the subject. The issue is also present in the original community edition gruvbox, and an issue is also open there gruvbox-community/gruvbox#173

Describe the solution you'd like
One solution is to change the diff highlight groups to only affect the background, keeping the foreground with syntax highlights. See #225 and #235 for implementation attempts.
This is the solution employed by several themes, like catpuccin
https://github.com/catppuccin/nvim/blob/main/lua/catppuccin/groups/syntax.lua#L74-L77
Catpuccin uses the same technique I used in #235 - blending the color with the background - except I did that offline and they do it in lua.

This is not to the taste of everyone, so an option can be added to set the feature.

Additional context
See #105, #225 and #235 for screenshots and previous discussions on the subject.

[polyzen]

#239

[Niverton]

#239

My bad, not really familiar with github I did not know the discussion section existed at all!