[RTL] Add support for character code BBCode tag [char=...].

[bruvzg]

Implements godotengine/godot-proposals#9481

[KoBeWi]

This will need to be documented here: https://docs.godotengine.org/en/stable/tutorials/ui/bbcode_in_richtextlabel.html#reference

[KoBeWi]

Works correctly.
I only wonder about the name. Is uni a standard BB code for unicode?

[bruvzg]

There's no standard bbcode for it (same is true for a some other tags we use).

[akien-mga]

Would [char=...] be more explicit to users maybe?

[coppolaemilio]

if there isn't a standard I would go with the full word [unicode=...] or with [char=...] like Akien suggested.
The short versions like [img] instead of [image] seem to only exists if there are html tags with the same name, if not, the full word is used (as far as I can tell, making it up tbh, so feel free to ignore).

[akien-mga]

Based on the RC discussion, let's go with [char=...] then.

[akien-mga]

Thanks!