Adding API to get field name of a TSNode

This PR adds an API to get name of the field of TSNode's child.
It uses same set of arguments as that of ts_node_child, but returns
field name if it is found, otherwise it returns NULL.

This API is useful to implement custom printing of S-expressions such
as following:

"(binary_expression
   (binary_expression_left (identifier))
   (binary_expression_operator ("+"))
   (binary_expression_right (identifier)
)"

Currently, ts_node_string does not allow any customization for printing.

lib/include/tree_sitter/api.h

@@ -487,6 +487,12 @@ TSNode ts_node_parent(TSNode);
  */
 TSNode ts_node_child(TSNode, uint32_t);
 
+/**
+ * Get the field_name for node's child at the given index, where zero represents
+ * the first child. Returns NULL, if no field is found.
+ */
+const char *ts_node_child_field_name(TSNode, uint32_t);
+
 /**
  * Get the node's number of children.
  */

lib/src/node.c

@@ -578,6 +578,23 @@ recur:
   return ts_node__null();
 }
 
+const char *ts_node_child_field_name(TSNode self, uint32_t child_index) {
+  const TSFieldMapEntry *field_map, *field_map_end;
+  ts_language_field_map(
+    self.tree->language,
+    ts_node__subtree(self).ptr->production_id,
+    &field_map,
+    &field_map_end
+  );
+
+  for (const TSFieldMapEntry *i = field_map; i < field_map_end; i++) {
+    if (i->child_index == child_index) {
+      return self.tree->language->field_names[i->field_id];
+    }
+  }
+  return NULL;
+}
+
 TSNode ts_node_child_by_field_name(
   TSNode self,
   const char *name,