refactor: allow not having a default case for enum

Problem: The style guide states that all switch statements that are not conditional on an enum must have a `default` case, but does not give any explicit guideline for switch statements that are conditional on enums. As a result, a `default` case is added in many enum switch statements, even when the switch statement is exhaustive. This is not ideal because it removes the ability to have compiler errors to easily detect unchanged switch statements when a new possible value for an enum is added.

Solution: Add explicit guidelines for switch statements that are conditional on an enum, clarifying that a `default` case is not necessary if the switch statement is exhaustive. Also refactor pre-existing code with unnecessary `default` cases.

src/nvim/eval/funcs.c

@@ -4990,9 +4990,10 @@ static int msgpackparse_convert_item(const msgpack_object data, const msgpack_un
     tv_list_append_owned_tv(ret_list, tv);
     return OK;
   }
-  default:
+  case MSGPACK_UNPACK_EXTRA_BYTES:
     abort();
   }
+  UNREACHABLE;
 }
 
 static void msgpackparse_unpack_list(const list_T *const list, list_T *const ret_list)