Rollup merge of #137606 - davidtwco:next-edition, r=traviscross,ehuss

add a "future" edition

This idea has been discussed previously [on Zulip](https://rust-lang.zulipchat.com/#narrow/channel/213817-t-lang/topic/Continuous.20edition-like.20changes.3F/near/432559262) (though what I've implemented isn't exactly the "next"/"future" editions proposed in that message, just the "future" edition). I've found myself prototyping changes that involve edition migrations and wanting to target an upcoming edition for those migrations, but none exists. This should be permanently unstable and not removed.

Author: jhpratt