Deprecate String.is_valid_identifier()

[timothyqiu]

String.is_valid_identifier() currently checks if the string is ascii identifier (0-9A-Za-z_, not starting with a digit). This does not match the current rule of GDScript identifier anymore.

I suggest

• Adding is_valid_ascii_identifier()
• Adding is_valid_unicode_identifier()
• Deprecating is_valid_identifier(), making it an alias of is_valid_ascii_identifier() for compatibility reason.

[akien-mga]

Implemented by godotengine/godot#95984.