Set up triagebot for ACPs

[clarfonthey]

Opening an issue here so we can nominate it.

Note that there would need to be a few changes made to triagebot to support ACPs as being different from MCPs, and we might want to decide whether triagebot should also automatically open tracking issues or not. (Which is currently done for MCPs.)

Relevant triagebot.toml for rust-lang/compiler-team: https://github.com/rust-lang/compiler-team/blob/master/triagebot.toml

[tgross35]

I like the idea of automatic tracking issues, but we probably get more ACPs from people without write access than compiler does MCPs. So they wouldn’t be able to update it with the final API signature and unresolved questions.

[clarfonthey]

That's a very good point, and I think that alone is enough to encourage ACP authors to file tracking issues themselves.