Add ts_node_language() that returns the language of the node

Since an input might have a nested sublanguage (like Javascript inside
Vue) and symbols are per-language, we need to know which language a node
is.

lib/src/node.c

@@ -423,6 +423,10 @@ const char *ts_node_type(TSNode self) {
   return ts_language_symbol_name(self.tree->language, symbol);
 }
 
+const TSLanguage *ts_node_language(TSNode self) {
+  return self.tree->language;
+}
+
 char *ts_node_string(TSNode self) {
   return ts_subtree_string(ts_node__subtree(self), self.tree->language, false);
 }