Right align scrollbar with track in completion popup

[sudormrfbin]

Depends on #2728.

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• Align the scollbar to the right edge of the popup rather than at a margin of one.
• Add a scrollbar track and a new scope ui.menu.scroll.

Before	After

A multiline scrollbar like the one mentioned in #2728 (comment) would be nicer though:

[archseer]

Changes the scrollbar block character to be thicker.

I preferred the thinner one! But rendering block chars seems to vary a lot by terminal

[sudormrfbin]

I was experimenting with adding a scroll background, and it looked better with a thicker bar. We can't style cells partially with another color, hence if a half block is used the selection style will look cut off:

Thin half bar	Thick full bar

The thin bar looks much better but the gap is nagging. Is the background something that we even want? If so which type of block character? Keeping it simple with a thin block (like earlier) is also easier to do.

[CptPotato]

The thin bar looks much better but the gap is nagging.

Isn't it possible to use the foreground and background colors of the half-block to fill the gap with the selection color? Or is the background color dictated by the window?

[sudormrfbin]

Got the thin block working without the annoying gap, thanks @CptPotato for the pointer!

A new scope ui.menu.scroll controls the colors of the the scrollbar thumb and the track.

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Note: The PR now depends on #2728.

[CptPotato]

Looks very clean, I like it!

[archseer]

Oh yeah the alignment was an off-by-1 error for sure 😆

[archseer]

Can you rebase?

[sudormrfbin]

Rebased!