Add typings modules to match UMD modules. (#3821)

* Add typings modules to match UMD modules.

typings/php.d.ts

@@ -0,0 +1,15 @@
+/**
+ * @license
+ * Copyright 2020 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/**
+ * @fileoverview Type definitions for the PHP generator.
+ * @author samelh@google.com (Sam El-Husseini)
+ */
+
+/// <reference path="core.d.ts" />
+
+import * as Blockly from './core';
+export = Blockly.Generator;