Extensions to label-to-title

[liamhuber]

• Proof-check that the label doesn't already start with "[TAG]" -- right now, if you add the "patch" label to a title already starting with "[patch]" you get "[patch] [patch] ...". IMO, ideal behaviour would be for it to not act if the title already starts with the new prefix, and to fail if it starts with a different prefix. Mostly just some extra bash work
• Run the action in reverse, i.e. if the label is updated and it starts with a "[TAG]", add the corresponding label
• Make it flexible, so that tags are space-separated input with defaults of "patch major minor" so it can add arbitrary title prefixes. Good practice manipulating action input, plus a bit of bash