Add String.is_valid_unicode_identifier()

[timothyqiu]

Implements godotengine/godot-proposals#9851

Rationale:

• The old is_valid_identifier() is checking for traditional ASCII identifiers. Its name can be confusing because GDScript implements Unicode identifiers.
• There is currently no way to check according to GDScript's rule of Unicode identifiers. TextServer.is_valid_identifier() is close, but advanced text server adds extra conversions and rules. Text server extensions could potentially also change the rules.

This PR:

• Adds is_valid_unicode_identifier(): Basic unicode identifier checking, the same as how GDScript parses identifiers
• Adds is_valid_ascii_identifier() : Same as the old is_valid_identifier()
• Deprecates is_valid_identifier()
• Renames internal method validate_identifier() to validate_ascii_identifier()

[Calinou]

This change makes sense to me. I've been wondering how to check for a valid Unicode identifier recently from a script, in fact 🙂

[Mickeon]

The first line of the new method's description could be a bit more detailed to entail what a "valid Unicode identifier" means, at least roughly.

The examples could also be more distinct than is_valid_identifier's, such as using accented letters and similar.

[akien-mga]

Thanks!