Fix TextServer::shaped_text_*_character_pos for the first character of wrapped string. Allow starting/ending RTL selection before line start.

[bruvzg]

• Fixes Can't select the first letter of a wrapped line in a RichTextLabel #95297
• Fixes Fix TextServer::shaped_text_*_character_pos for the first character of wrapped string. Allow starting/ending RTL selection before line start. #95301
• Fixes selection of the wrapped line started before start of the line (see [indent] example in Can't begin a text selection outside the bounds of a RichTextLabel #95298).

[eviltrout]

I tested this change locally and can confirm it completely solves the issues. I tested with indents and without, with content margins too. Works great. Thank you so much @bruvzg!

[kitbdev]

• related Fix TextEdit caret movement at start of wrapped lines #91978
• fixes Can't select first character with mouse in script editor #88632
• supersedes Fix selecting first character with mouse in script editor #88813

I can change my #91978 to only be about caret movement fixes after this.

[akien-mga]

Thanks!

[akien-mga]

Cherry-picked for 4.3.1.