[RTL] Add support for vertical alignment.

[bruvzg]

Adds vertical_alignment property.

Closes godotengine/godot-proposals#6674

[Calinou]

Note that this PR doesn't implement godotengine/godot-proposals#4458, which asks for BBCode tags to control alignment on a per-paragraph basis. This may be too complex to implement though, and most use cases seem to only require a per-RichTextLabel property to control vertical alignment.

There could be a horizontal alignment property, so that you can center text without needing to always wrap it in [center]. BBCode tags would still be able to override it, so it'd act as a default for paragraphs that don't specify their alignment. This is likely better implemented in its own PR since it's technically independent.

[Calinou]

Tested locally (including with different font sizes in the RichTextLabel), it works as expected.

[akien-mga]

Thanks!