"Generate `Deref` impl" should also offer generating a `DerefMut` impl

[jonas-schievink]

struct S {
    target$0: u8,
}

Here we're offering a "Generate Deref impl using target" assist. This currently only supports a Deref impl though, not a DerefMut impl that you might also want.

It should either open a popup where it asks the user (like the "generate getter/setter" assist), or an assist should be offered on the Deref impl to generate an identical (except with &mut instead of &) DerefMut impl.

[jhgg]

Yeah... I was going to do this but never got around to it lol...