Add TreeCursor.currentFieldName to wasm binding

lib/binding_web/test/tree-test.js

@@ -263,6 +263,34 @@ describe("Tree", () => {
       assert(!cursor.gotoParent());
     });
 
+    it('keeps track of the field name associated with each node', () => {
+      tree = parser.parse('a.b();');
+      cursor = tree.walk();
+      cursor.gotoFirstChild();
+      cursor.gotoFirstChild();
+
+      assert.equal(cursor.currentNode().type, 'call_expression');
+      assert.equal(cursor.currentFieldName(), null);
+
+      cursor.gotoFirstChild();
+      assert.equal(cursor.currentNode().type, 'member_expression');
+      assert.equal(cursor.currentFieldName(), 'function');
+
+      cursor.gotoFirstChild();
+      assert.equal(cursor.currentNode().type, 'identifier');
+      assert.equal(cursor.currentFieldName(), 'object');
+
+      cursor.gotoNextSibling();
+      cursor.gotoNextSibling();
+      assert.equal(cursor.currentNode().type, 'property_identifier');
+      assert.equal(cursor.currentFieldName(), 'property');
+
+      cursor.gotoParent();
+      cursor.gotoNextSibling();
+      assert.equal(cursor.currentNode().type, 'arguments');
+      assert.equal(cursor.currentFieldName(), 'arguments');
+    });
+
     it('returns a cursor that can be reset anywhere in the tree', () => {
       tree = parser.parse('a * b + c / d');
       cursor = tree.walk();