Add LSP action to automatically import a trait when using one of its methods

[TomAFrench]

In #6882 we added a warning for situations where we're using a trait but it's not in scope. For the case where there's only a single candidate trait we can then automatically import it. This would make it much easier to update any legacy codebases (and generally be more ergonomic on new ones).

Rust also allows selecting from multiple candidate traits which would also be nice but can be done separately.

[asterite]

Relatedly, when typing x. and the completion is a trait method, the trait method is inserted but not the corresponding import (because of how it used to work). So now we'd also need to add the corresponding import. I'll try to do both things in the same PR as they are kind of related.