This further reduces the number of things declared in the public
regexp API file, currently still named jsregexp.h.

* Move JSRegExp::Flags convenience functions to regexp-compiler.h.
* Set RegExpImpl methods private if possible (these will later be
  moved to a new hidden impl class).
* Merge RegExpEngine::CompilationResult into RegExpCompileData.
* Move remaining RegExpEngine methods to RegExpImpl and delete
  RegExpEngine.
* Extract RegExpGlobalCache.
* Document a few data structures.

Upcoming CLs will rename RegExpImpl to RegExp and jsregexp.h to
regexp.h. This should then be the only header included from other
directories.

Bug: v8:9359
Change-Id: I78c8f4cca495a2b95735a48b6181583bc3310bdf
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1662294Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62218}