Add a code action to replace `_` with type

[giacomocavalieri]

It would be nice if the LS allowed to replace a _ anywhere in a type annotation with the corresponding type. Many times I start by writing some incomplete type Result(_, _) and fill in the blanks later, right now to do that I'd delete the entire signature and have the LS generate it again.

So for example:

pub fn wibble() -> Result(List(_), Nil) {
//             trigger it here ^
  Ok([1, 2, 3])
}

produces:

pub fn wibble() -> Result(List(Int), Nil) {
  Ok([1, 2, 3])
}