Disallow creating new labels

[sarina]

What you want to add

A flag that would give the option to disallow creating new labels.

Why this is needed

I added a new action that allows users to add labels, but they can add arbitrary labels. I'd like to restrict them to adding only labels already on the repo.

Implementation notes

I may have time to tackle this later, but my JS skills are poor. It seems like around here: https://github.com/actions-ecosystem/action-add-labels/blob/main/dist/index.js#L2046-L2052 you add a call to https://github.com/actions-ecosystem/action-add-labels/blob/main/dist/index.js#L12982-L13001 and check if each new label is in the repo, and if not throw it away. Not sure the best way to raise an error or call out the discarded labels to the user.