[Python] Improve named backref regexp highlighting

[deathaxe]

This commit prepares named backreferences for correctly handling interpolation.

Primary use case are raw f-strings, but it applies to all other sorts of template languages as well.

Notes:

1. to align with Regular Expressions package, keyword.other.backref-and-recursion covers all, leading ? and group names.
2. adds ?&backref to support patterns from "regexp" package.

[michaelblyons]

Looks good.

Would interpolations still work if you added an invalid.illegal for [^:[alnum]:_] in backref-name? (Or are there other concerns for annoyance while typing? I would expect any nonconforming characters to be inside the interpolations and not really a problem if interpolations are working.)

[deathaxe]

With regards to related Discord discussion, it may require additional messures to ensure place holders like %s to not be highlighted illegal. Those are currently not included into with_prototype in Python.

A point of concern is [^:[alnum]:_] pattern creating individual tokens for each character and [^:[alnum]:_]+ potentially preventing interpolation from being highlighted, when preceeded by an non-word character. But maybe individual tokens being created may be unlikely and expected ammount small enough to be acceptable.

It doesn't currently produce good results, as normal patterns don't handle placeholders as those cause ambiguities with {1} quantifiers. Adding illegals results in ...

With current PR, they at least don't look too broken.