Highlighter based on Matches API [LUCENE-8349]

I started trying to integrate the Matches API into the UnifiedHighlighter, but there's a fairly heavy impedance mismatch between the way the two of them work (eg Matches doesn't give you freqs, it's entirely lazy, the UH tries to do things by field rather than by doc). So instead, I thought I'd try and write a new highlighter based around Matches, and see what it looks like.

---
Migrated from [LUCENE-8349](https://issues.apache.org/jira/browse/LUCENE-8349) by Alan Woodward (@romseygeek), updated Jun 21 2018