Clarify visible_characters

[Joy-less]

See: godotengine/godot-proposals#12654

This clarifies the behaviour of Label.visible_characters and RichTextLabel.visible_characters with regards to UNICODE. From my tests and the collaborator response, for all visible_characters_behavior options, a character is considered to be a UNICODE code point (A.K.A. a UTF-32 character), which may be inconsistent with the visible graphemes/runes and with C# UTF-16 strings.

[bruvzg]

Looks fine, this is how it works right now. We have code to detect grapheme clusters (used to control the cursor in text edit nodes), so we can eventually change this behavior if desired. But this will be a breaking change.