Add a fix that inserts a missing method

[MatrixFrog]

Work in progress! Not ready for review yet.

Often I will call a method even though I know it doesn't exist yet, because I'm planning to add that method.

struct Dog {}

impl Dog {
  bark(&self) {}
}

fn main() {
  let dog = Dog {};
  dog.bark();
  dog.play_dead();  // errors because the method doesn't exist yet
}

This PR adds a fix that inserts a stub for the play_dead method.

• For now, the receiver type is &self but maybe we can figure out whether it should be self or &mut self instead?
• Return type is not added but it could be inferred if the calling code looks like let lhs: SomeType = dog.play_dead()
• Should the method body be todo!() or unimplemented!() or something else? What is the most consistent with how r-a does things elsewhere?
• Need to do more testing

[ChayimFriedman2]

Should the method body be todo!() or unimplemented!() or something else? What is the most consistent with how r-a does things elsewhere?

There is a config for what to insert in e.g. fill match arms, I think it's appropriate to use it here as well.

[MatrixFrog]

this test currently fails. we're calling a non-existent method on &Dolphin (not Dolphin) so no fix is generated. In such a case we should look for an impl Dolphin block and add a method that takes &self, or look for an impl &Dolphin block and add a method that takes self. What if both exist?

[MatrixFrog]

Haven't yet tested cases with &mut SomeType but probably need to handle that too.

[Veykril]

There is an assist that can create functions from unresolved ones,

rust-analyzer/crates/ide-assists/src/handlers/generate_function.rs

Line 43 in ab9e7bd

// ```

Naturally, that should turn into a diagnostic in the long run (when we have more confident diagnostics for this).

Would be good to deduplicate code here, especially as the assist already supports this use case better (being signature aware)