feat: add scrollbar background color

[Angelchev]

Add configurable scrollbar background color

Close #52

• [x] Add configurable scrollbar background highlight color via SatelliteBackground highlight group. Uses the ColorColumn highlight group by default.
• [x] Update docs

Potential Issues

• Background color may conflict with mark highlights.
• The additional bar for marks not combined in the scrollbar is not updated. Although I notice the bar highlight representing the current view of the buffer is not used there also.

Screenshots

Default (ColorColumn)

Custom

White	White & Winblend 50	Cyan	Cyan & Winblend 50	Pink	Pink & Winblend 50

[Angelchev]

After some additional testing on a GUI like neovide I have encountered some highlighting issue that may need fixing:

Scrollbar center highlighting should be SatelliteBar instead of the SatelliteBackground.

Screenshot (Cyan background for clarity)

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Will look into fixing this soon.

[Angelchev]

I have double-checked and it looks like the above issue was just due to some local dev changes. It works just fine in the PR:

Neovide screenshot (Cyan background for clarity)

As you can see the background color of the extended marks for git is not changed, but I personally think this is OK, or can be added later.

@lewis6991 feel free to merge if happy :)

[lewis6991]

By default I'd prefer the background to be transparent like it is now.