feat: adds hooks for serializing plugins (#5276)

* Reformat registry tests

* Add tests for plugin hooks

* Add plugin hooks for serialization

* Switch PluginSerializer to IPluginSerializer

* fix: types

* fix: PR comments

* fix: tests

* cleanup: formatting

* fix: types

* feat: add respecting case in registry

* feat: add separate registry for serializers

* fix: rename serialiation registry alias

* fix: move serializer interface into interface dir

core/interfaces/i_serializer.js

@@ -0,0 +1,71 @@
+/**
+ * @license
+ * Copyright 2021 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/**
+ * @fileoverview The record type for an object containing functions for
+ *     serializing part of the workspace.
+ */
+
+'use strict';
+
+goog.module('Blockly.serialization.ISerializer');
+goog.module.declareLegacyNamespace();
+
+// eslint-disable-next-line no-unused-vars
+const Workspace = goog.requireType('Blockly.Workspace');
+
+
+/**
+ * Serializes and deserializes a plugin.
+ * @interface
+ */
+class ISerializer {
+  constructor() {
+    /**
+     * A priority value used to determine the order of deserializing plugins.
+     * More positive priorities are deserialized before less positive
+     * priorities. Eg if you have priorities (0, -10, 10, 100) the order of
+     * deserialiation will be (100, 10, 0, -10).
+     * If two plugins have the same priority, they are deserialized in an
+     * arbitrary order relative to each other.
+     * @type {number}
+     */
+    this.priority;
+  }
+
+  /* eslint-disable no-unused-vars, valid-jsdoc */
+
+  /**
+   * Saves the state of the plugin.
+   * @param {!Workspace} workspace The workspace the plugin to serialize is
+   *     associated with.
+   * @return {?} A JS object containing the plugin's state, or null if
+   *     there is no state to record.
+   */
+  save(workspace) {}
+
+  /* eslint-enable valid-jsdoc */
+
+  /**
+   * Loads the state of the plugin.
+   * @param {?} state The state of the plugin to deserialize. This will always
+   *     be non-null.
+   * @param {!Workspace} workspace The workspace the plugin to deserialize is
+   *     associated with.
+   */
+  load(state, workspace) {}
+
+  /**
+   * Clears the state of the plugin.
+   * @param {!Workspace} workspace The workspace the plugin to clear the state
+   *     of is associated with.
+   */
+  clear(workspace) {}
+
+  /* eslint-enable no-unused-vars */
+}
+
+exports.ISerializer = ISerializer;