feature: Palette overrides

[arkrost]

Describe the solution you'd like

Please add an option to override palette colours. Right now, I usually use before_highlight to make my adjustments:

require('rose-pine').setup({
before_highlight = function(group, highlight, palette)
   if highlight.fg == palette.pine then
     highlight.fg = "#XXXXXX"
   end
   ...
end
})

However, I'd prefer to have it as a standard option and have ability to add variants for all theme variants.

Additional context

No response

[mvllow]

Thank you for showing interest in this feature. It's on my radar :)

[mvllow]

Completed in #307 :)