Implement the alternative try block desugaring

[scottmcm]

As discussed in rust-lang/rfcs#3721 (comment), update the try in nightly to match the RFC as a way to experiment.

This addresses the following unresolved issue from #31436

Address issues with type inference (try { expr? }? currently requires an explicit type annotation somewhere).

[rustbot]

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[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.