feat(autocmds): retrieve lua callback (#18642)

add a new `callback` field to `nvim_get_autocmds`

src/nvim/eval/typval.c

@@ -1205,6 +1205,30 @@ void callback_copy(Callback *dest, Callback *src)
   }
 }
 
+/// Generate a string description of a callback
+char *callback_to_string(Callback *cb)
+{
+  size_t msglen = 100;
+  char *msg = (char *)xmallocz(msglen);
+
+  switch (cb->type) {
+  case kCallbackLua:
+    snprintf(msg, msglen, "<lua: %d>", cb->data.luaref);
+    break;
+  case kCallbackFuncref:
+    // TODO(tjdevries): Is this enough space for this?
+    snprintf(msg, msglen, "<vim function: %s>", cb->data.funcref);
+    break;
+  case kCallbackPartial:
+    snprintf(msg, msglen, "<vim partial: %s>", cb->data.partial->pt_name);
+    break;
+  default:
+    snprintf(msg, msglen, "%s", "");
+    break;
+  }
+  return msg;
+}
+
 /// Remove watcher from a dictionary
 ///
 /// @param  dict  Dictionary to remove watcher from.