Fix precedence of ui.virtual.whitespace

[chtenb]

This fixes the first half of #5675, namely the whitespace case.
The idea is to move the whitespace highlighting logic to the same place as the other highlighting logic, such that it's precedence can be precisely controlled.

[pascalkuthe]

I donüt think we should implement it this way. That kind of highlighting is fairly inefficient since it requires collection to a vec (its fine for something rare like the cursor but not something common like whitespace). Even worse it requires a secondary transversal of all visible text. The text transversal is already the rendering bottleneck so doing it twice is not a good idea.

I am still not sure what the best way to handle this would be without causing too much overhead. One way could be to maintain two highlgt iterators. One for sytnax highlighting and one for overlays. The we could have the option to patch highlights before the syntax highlts, after the syntax highlights but before the overlay (like whitespace and indent guides) and then finally after the overlays.

That also should be pretty easy to implement.