feat: add JSO serialization and deserialization of variables (#5131)

* Add variable serialization tests

* Fix requires for new file

* Add serialization and deserialization of variables

* Remove only in tests

* Cleanup

core/serialization/variables.js

@@ -0,0 +1,62 @@
+/**
+ * @license
+ * Copyright 2021 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/**
+ * @fileoverview Handles serializing variables to plain JavaScript objects, only
+ *     containing state.
+ */
+'use strict';
+
+goog.module('Blockly.serialization.variables');
+goog.module.declareLegacyNamespace();
+
+// eslint-disable-next-line no-unused-vars
+const VariableModel = goog.requireType('Blockly.VariableModel');
+// eslint-disable-next-line no-unused-vars
+const Workspace = goog.requireType('Blockly.Workspace');
+
+
+/**
+ * Represents the state of a given variable.
+ * @typedef {{
+ *   name: string,
+ *   id: string,
+ *   type: (string|undefined),
+ * }}
+ */
+var State;
+exports.State = State;
+
+/**
+ * Returns the state of the variable as a plain JavaScript object.
+ * @param {!VariableModel} variableModel The variable to serialize.
+ * @return {!State} The serialized state of the variable.
+ */
+const save = function(variableModel) {
+  const state = {
+    'name': variableModel.name,
+    'id': variableModel.getId()
+  };
+  if (variableModel.type) {
+    state['type'] = variableModel.type;
+  }
+  return state;
+};
+/** @package */
+exports.save = save;
+
+/**
+ * Loads the variable represented by the given state into the given workspace.
+ * Do not call this directly, use workspace.createVariable instead.
+ * @param {!State} state The state of a variable to deserialize into the
+ *     workspace.
+ * @param {!Workspace} workspace The workspace to add the variable to.
+ */
+const load = function(state, workspace) {
+  workspace.createVariable(state['name'], state['type'], state['id']);
+};
+/** @package */
+exports.load = load;