Add PluginManager (#4672)

* Add PluginManager.

core/interfaces/i_workspace_plugin.js

@@ -0,0 +1,20 @@
+/**
+ * @license
+ * Copyright 2021 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/**
+ * @fileoverview Interface for plugins that can be registered on the workspace.
+ * @author kozbial@google.com (Monica Kozbial)
+ */
+
+'use strict';
+
+goog.provide('Blockly.IWorkspacePlugin');
+
+/**
+ * Base interface for a plugin that can be registered on the workspace.
+ * @interface
+ */
+Blockly.IWorkspacePlugin = function() {};