Don't suggest unstable items on stable toolchain
Closes#3020
This PR implements stability check in `ide-completion` so that unstable items are only suggested if you're on nightly toolchain.
It's a bit unfortunate `CompletionContext::check_stability()` is spammed all over the crate, but we should call it before building `CompletionItem` as you cannot get attributes on the item it's completing from that struct. I looked up every callsite of `Builder::add_to()`, `Completions::add[_opt]()`, and`Completions::add_all()` and inserted the check wherever necessary.
The tests are admittedly incomplete in that I didn't add tests for every kind of item as I thought that would be too big and not worthwhile. I copy-pasted some existing basic tests in every test module and adjusted them.
fd276218ec