feat: Add destructure_enum assist

[Wilfred]

r-a already has the ability to fill in match arms as part of the add_missing_match_arms assist. However, there's currently no way to go from flavor to match flavor { Apple => {} ... }.

If a user does want the IDE to fill in match arms, they have to write the match and {} themselves. Other language LSPs supporting pattern matching (e.g. merlin for OCaml) support writing the whole match statement.

Whilst I'm aware there's a freeze on new assists, this assist is just a generalisation of add_missing_match_arms and reuses the existing logic there (which has great test coverage). I think it's a really compelling feature and hopefully a small ask.

AI disclosure: Written with a little bit of help from Claude in a few places.

[A4-Tacks]

Currently, for Option and Result, postfix completion overlaps with this assistant

Perhaps we should strengthen postfix completion instead of adding assistants?

Related #11409

crates/ide-completion/src/completions/postfix.rs:165

[Wilfred]

Huh, I had totally forgotten about .match. Thanks for the issues too: in addition to #11409, I think the linked #8365 is very relevant. This PR is essentially adding case splitting (maybe that's a better name for this assist?).

The downsides of .match is that it's much harder to discover (not valid Rust syntax and I don't think there's precedent in other languages?) and it's awkward to use when you have larger expressions. For example, if I have foo.bar.baz I can highlight the whole expression, highlight foo.bar or just highlight foo and destructure depending on the type of those subexpressions.