Closes #33106

Instead of directly using `filter_text` in `StringMatchCandidate`, which
yields better results for fuzzy matching but messes up with highlighting
letters in bold, as `positions` list generated by fuzzy crate are now of
`filter_text` and not `label` shown to the user.

This PR fixes it by keeping use of `filter_range` in
`StringMatchCandidate`, which is range w.r.t to `label` shown to user.
And actually generating this `filter_range` at source by using
`filter_range` if exists.

- [x] Tests

Release Notes:

- Fixed issue where incorrect letters are marked as bold in completions.