Edition::EditionFuture for forwards compatibility

[cormacrelf]

Like #21423 but for editions.

I would like it so that RA is not necessarily bricked as soon as a new edition comes out. Usually a new edition means RA will have to update (new syntax, new keywords, new trait stuff, etc) but we should allow it to lurch on a little longer if that is not bothering a given user. I have been forced to upgrade because of this and it would have been nice at the time to do that the following month instead of in a mega-upgrade of rustc, 100 crates, rust-analyzer, and buck2 all at once.

I'm not sure if this is the exact right approach. Maybe we just assume all editions will be years and parse the string to a number? Then we can at least inject the prelude using a non-interned symbol to construct a use ::std::prelude::rust_xxxx::* import, rather than falling back to the latest edition and probably missing some symbols.

[ChayimFriedman2]

I definitely do not want us to introduce a new "fake" variant to the main Edition enum. I do support, though, deserializing unknown editions into the oldest or latest edition - whatever people prefer (and assuming the consensus on #21423 will be positive).