Restrict identifier grammar

[straight-shoota]

This patch restricts the grammar for identifiers to the suggested class Default Identifiers from UAX #31 (revision 35).

Also adds character data for the Unicode general category Pc and a couple of nodoc methods for querying specific Unicode categories.

Resolves #11216

[beta-ziliani]

Only one little concern, the rest is 👌

[Blacksmoke16]

From discord, someone may have a found a regression:

:▲    # => Error: unexpected token: ":"
pp :▲ # => Error: unknown token: '▲'

I'm assuming this is what was meant by #11216, but wanted to at least report it to be sure.

[HertzDevil]

Symbol literals should accept a larger grammar than identifiers, since they already allow things like :+ after all, and + is definitely not a valid identifier.

[straight-shoota]

I suppose, for symbols we can probably use ident_part? even for the first character, because the : prefix is already a proper start designation.

[straight-shoota]

However, U+25B2 (▲) is actually in general category So (Other Symbol), so it wouldn't even be valid for ident_part?.