Update highlights

[ttytm]

If that fits to the repo, the highlights groups could be updated / extended.

Highlights for diagnostic could link to the nvim highlights groups DiagnosticSignWarn, DiagnosticSignError etc.. Those are also the ones that are usually manipulated by themes. It might make this part of barbar a little more cross theme compatible out of the box.

Groups that could be added related to 'Buffer' .. activity .. 'Sign' are 'Buffer' .. activity .. 'SignLeft' and 'Buffer' .. activity .. 'SignRight'. This is something for the tinkerers. For me personally it would allow me to keep a style I made for my bufferlines that emulates a shadow thrown from active buffers on other buffers / the tabline. I called this style "slim shady" back then lol.

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edit: I see that the first suggestion with linking the groups would probably interfere with the background. So i'd like to update the suggestion to deriving the fg color for those groups from the mentioned nvim hl groups.

[Iron-E]

Related: #199

[Iron-E]

@tobealive does #485 help?

[ttytm]

awesome, thanks @Iron-E !