Add Comments::find_before

[LucaCappelletti94]

Comments::find_before(location) returns the comments between a token and the previous non-whitespace source token, in source order. It provides the token-relative lookup discussed in #2065 while leaving comment association to callers.

CommentWithSpan records prev_token_end and next_token_start for the lookup and caller-defined association. These additional fields are a breaking API change for code constructing CommentWithSpan directly.

[github-actions]

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.34%. Comparing base (d32825d) to head (69273e4).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2691      +/-   ##
==========================================
+ Coverage   81.29%   81.34%   +0.04%     
==========================================
  Files          42       42              
  Lines       33950    34041      +91     
  Branches    33950    34041      +91     
==========================================
+ Hits        27601    27692      +91     
  Misses       2785     2785              
  Partials     3564     3564              

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