Merge #6251 #6310

6251: Semantic Highlight: Add Callable modifier for variables r=matklad a=GrayJack

This PR added the `HighlightModifier::Callable` variant and assigned it to variables and parameters that are fn pointers, closures and implements FnOnce trait.

This allows to colorize these variables/parameters when used in call expression.



6310: Rewrite algo::diff to support insertion and deletion r=matklad a=Veykril

This in turn also makes `algo::diff` generate finer diffs(maybe even minimal diffs?) as insertions and deletions aren't always represented as as replacements of parent nodes now.

Required for #6287 to go on.

Co-authored-by: GrayJack <[email protected]>
Co-authored-by: Lukas Wirth <[email protected]>