UnifiedHighlighter: internally visit the query tree once [LUCENE-9277]

Ideally the UnifiedHighlighter should "visit" the query tree **once** instead of several times (weight.extractTerms, MultiTermHighlighting, PhraseHelper). Perhaps this can happen in one new class, perhaps called QueryExtractor. It's debatable wether this would replace a bunch of fields presently on UHComponents or whether it would simply help produce the existing UHComponents; shrug.

Admittedly, I don't know how much of an "optimization" this is, or wether this is just a refactoring that is done on principle. I simply like the principle of it; knowing there are multiple _visit_s to the query gnaws at me.

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Migrated from [LUCENE-9277](https://issues.apache.org/jira/browse/LUCENE-9277) by David Smiley (@dsmiley)

Start by tracing UnifiedHighlighter and UHComponents, then compare the query-tree visits in weight.extractTerms, MultiTermHighlighting, and PhraseHelper. Determine whether a QueryExtractor can unify those visits without changing highlighting behavior; done means the query tree is visited once and the existing UHComponents behavior remains correct.

Written by the indexing model from the issue text.