Add PluginManager (#4672)

* Add PluginManager.

core/interfaces/i_plugin.js

@@ -0,0 +1,22 @@
+/**
+ * @license
+ * Copyright 2021 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/**
+ * @fileoverview Interface for a plugin.
+ * @author kozbial@google.com (Monica Kozbial)
+ */
+
+'use strict';
+
+goog.provide('Blockly.IPlugin');
+
+
+/**
+ * The interface for a workspace plugin.
+ * @interface
+ */
+Blockly.IPlugin = function() {};
+