Please make the HighlightQuery class public

We're using the highlighter directly in our code, and the introduction of the package-scoped HighlightQuery in 4aa59aff0058b05e822592431eda4a469a6b9eef has made this impossible without building our own little fork.
@javanna We actually subclassed `HighlighterContext` and don't use `HighlightPhase`. If the constructor is not public, we can't instantiate a HighlightQuery and can't really initialize our subclass properly.
There might be other ways to achieve our goals, but like I said, it worked for us before and this change seems like the simplest thing that works.