feat: highlighting of return values while the cursor is on match / if / =>

[roife]

Inspired by #18924, a follow-up to #17542.

This PR adds a new feature that highlights the related return values when the cursor is on match, if, or a match arm arrow (=>). This helps users quickly identify the different possible outcomes of control flow expressions.

For match keyword:

• The match keyword itself
• All return expressions in each arm of the match

For => (match arm arrow):

• The arrow itself
• The return expression for that specific arm

For if keyword:

• All if keywords in the if-else chain
• All return expressions in the if-else branches

This feature works with nested expressions and macros.

[roife]

Here's a demo.

2025-04-09.10.32.29.mov

[roife]

I've been busy with work lately... Apologize for the delayed response.

[Veykril]

Thanks