add code action to replace underscore with corresponding type

[giacomocavalieri]

This PR closes #5447

//                trigger here! v
-pub fn wibble() -> Result(List(_), String) {
+pub fn wibble() -> Result(List(Int), String) {
  Ok([1 + 1])
}

---

• The changes in this PR have been discussed beforehand in an issue
• The issue for this PR has been linked
• Tests have been added for new behaviour
• The changelog has been updated for any user-facing changes