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() {};
+

core/interfaces/i_positionable.js

@@ -13,21 +13,19 @@
 
 goog.provide('Blockly.IPositionable');
 
+goog.require('Blockly.IPlugin');
+
 
 /**
  * Interface for a component that is positioned on top of the workspace.
+ * @extends {Blockly.IPlugin}
  * @interface
  */
 Blockly.IPositionable = function() {};
 
 /**
  * Positions the element. Called when the window is resized.
- * @param {!Blockly.MetricsManager.ContainerRegion} viewMetrics The workspace
- *     viewMetrics.
- * @param {!Blockly.MetricsManager.AbsoluteMetrics} absoluteMetrics The absolute
- *     metrics for the workspace.
- * @param {!Blockly.MetricsManager.ToolboxMetrics} toolboxMetrics The toolbox
- *     metrics for the workspace.
+ * @param {!Blockly.MetricsManager.UiMetrics} metrics The workspace metrics.
  * @param {!Array<Blockly.utils.Rect>} savedPositions List of rectangles that
  *     are already on the workspace.
  */
@@ -39,5 +37,3 @@ Blockly.IPositionable.prototype.position;
  * @returns {!Blockly.utils.Rect} The plugin’s bounding box.
  */
 Blockly.IPositionable.prototype.getBoundingRectangle;
-
-

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() {};