Assign team member who proposed fcp

[tmandry]

I found it really helpful when @dtolnay assigned me to https://github.com/rust-lang/rust/pull/115333 after I proposed fcp merge. This caused the PR notifications to show up in my "assigned" category and reinforced that I was the lang team member "shepherding" it through.

Additionally, when we manually merge RFCs and PRs after FCP is complete, we usually leave it up to the person who started the FCP.

I think we should automate and formalize this by having triagebot assign the FCP proposer.

[RalfJung]

Additionally, when we manually merge RFCs and PRs after FCP is complete, we usually leave it up to the person who started the FCP.

Do we? I was involved in several FPCs on implementation-heavy PRs. Review is usually done by someone from the compiler team, but it needs t-lang FCP so the FCP proposer is a t-lang member.

I think there are many PRs where assigning the FCP proposer does not make a lot of sense.