Parsed Regexp utilities

[marcandre]

We are starting to have more Cops handling Regexp. This PR

• simplifies the access to the regexp parsed tree with RegexpNode#parsed_tree
• caches to avoid reparsing the same node multiple times
• adds RegexpNode#each_capture which simplifies processing.

This could be helpful for #8504 too.

I'm hesitating between adding it to rubocop-ast or extending it as in this PR.

[bbatsov]

Won't we end up duplicating functionality from https://github.com/ammar/regexp_parser ?

[marcandre]

Right, I thought of making a PR for the gem itself; there are a few things that are annoying with the current API, etc.

In any case, RegexpNode#each_capture seems like a nice addition; maybe in the future it can be refactored to delegate the call to the gem?

[bbatsov]

In any case, RegexpNode#each_capture seems like a nice addition

I agree.

maybe in the future it can be refactored to delegate the call to the gem?

Yeah, that sounds reasonable.