Add new_draft kind of labels in [autolabel]

[Urgau]

This PR adds a new kind new_draft for labels in [autolabel], this is so that we can handle S-waiting-on-author in a similar way to S-waiting-on-review (ie add it when the PR is in draft state and remove it when it's not).

[autolabel."S-waiting-on-author"]
new_draft = true

Fixes #2102

[Kobzol]

Thanks for taking a look! I wonder if we should just have some "label anti-matter", where we would say that label A is mutually exclusive with label B 😆

This will also set new draft PRs to be in the waiting on author status, I wonder if it won't be confusing to some people. Anyway, we'll see in practice as usually. This will need documentation in the Forge an a rustc PR, as usually.

[Urgau]

we would say that label A is mutually exclusive with label B

I don't think it would necessarily help as removing a label (e.g. S-waiting-on-review) wouldn't add S-waiting-on-author, only adding S-waiting-on-author would remove S-waiting-on-review, right?

I say this as we have many exclusive labels around S-waiting-on-*.

In any case let's merge this as I think it's useful even if we later add an exclusive mechanism.

[jieyouxu]

FWIW, I don't think it's possible to enforce exclusive labels, because can't you manually apply both labels at the same time?