Switch to weight matches in the Unified highlighter

[jimczi]

The unified highlighter has a new mode that uses the matches API to perform highlighting:
https://issues.apache.org/jira/browse/LUCENE-8286
The main benefit of this mode is that it highlights boolean query accurately. Should we activate this mode by default ? This would be breaking since highlighting would be different so we could also add an option in 6x to activate this mode.

[elasticmachine]

Pinging @elastic/es-search-aggs

[jimczi]

We discussed internally and we've decided to create a new highlighter that will leverage the Matches API to perform highlighting. We don't want to add a new flag to the unified highlighter that would completely change the logic of the output so a new highlighter seems a better option. Some options will not be available due to how the logic is implemented, so for instance require_field_match would always be true since the Matches API ensures that the highlighted part are real "matches" in the document. I am closing this issue but opened #34015 to track the addition of this new highlighter.