Add padding options in theme for fg/bgcolor tags in RichTextLabel

[golfinq]

Addresses godotengine/godot-proposals#4808 by adding two new theme constants text_highlight_h_padding and text_highlight_v_padding which adds padding equally around the boxes the RichTextLabel tags fgcolor and bgcolor draw.

Example:

[KoBeWi]

Looks fine. It's a simple change.

[akien-mga]

Thanks!