Refactor completion doc popup and fix positioning

[sudormrfbin]

Refactor the logic for rendering completion documentation popup. The changes include using an early return, renaming some variables (overshadowing area, height, etc tends to be confusing) and making some area calculations more readable. Most importantly this PR also contains a fix for preventing the rendering of the completion doc popup over the completion popup itself and to not draw over the cursor.

Before	After

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Each commit does a single refactor and the last commit contains the bug fix (The full diff across all the commits might be hard to read since the first commit introduces an early return that dedents a whole block; stepping through the commits should be easier for reviewing).

[pascalkuthe]

Right now we render the documentation popup on the right of the completion popup when there is enough space. Maybe instead of showing the completion popup on top when there isn't enough space, we should show it in the left instead.
I think vscore does but that way too (and actually defaults to the left but that's not that importent).

[sudormrfbin]

I actually wanted to implement it but forgot about it in the middle of fixing the bug, thanks for the reminder :) Should be a pretty small change.

[archseer]

Great work!