Add horizontal alignment support to RichTextLabel

[jessicamaybe]

Revives #6472
Changes are basically the same

need this to add ss13 style chat to upstream

example:
https://github.com/jessicamaybe/space-station-14/tree/2026-08-12-richtextalignexample

setting the speech bubble alignment to Right

changed paper alignment to center with varying markups

markup in paper

Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod [head=1]tempor incididunt[/head] ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation [bold][head=3]ullamco laboris nisi ut aliquip ex[/bold][/head] ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.