[HTML5] PWA service worker prefers cached version.

Use an offline first approach, where we prefer the cached version over
the network one.
This forces games using PWA to always re-export the project and not just
the PCK, so that the service worker version gets updated correctly, and
the end-user cache is correctly cleared on update.

platform/javascript/js/engine/config.js

@@ -106,6 +106,13 @@ const InternalConfig = function (initConfig) { // eslint-disable-line no-unused-
 		 * @default
 		 */
 		experimentalVK: false,
+		/**
+		 * The progressive web app service worker to install.
+		 * @memberof EngineConfig
+		 * @default
+		 * @type {string}
+		 */
+		serviceWorker: '',
 		/**
 		 * @ignore
 		 * @type {Array.<string>}
@@ -249,6 +256,7 @@ const InternalConfig = function (initConfig) { // eslint-disable-line no-unused-
 		this.persistentDrops = parse('persistentDrops', this.persistentDrops);
 		this.experimentalVK = parse('experimentalVK', this.experimentalVK);
 		this.focusCanvas = parse('focusCanvas', this.focusCanvas);
+		this.serviceWorker = parse('serviceWorker', this.serviceWorker);
 		this.gdnativeLibs = parse('gdnativeLibs', this.gdnativeLibs);
 		this.fileSizes = parse('fileSizes', this.fileSizes);
 		this.args = parse('args', this.args);

platform/javascript/js/engine/engine.js

@@ -189,6 +189,9 @@ const Engine = (function () {
 							preloader.preloadedFiles.length = 0; // Clear memory
 							me.rtenv['callMain'](me.config.args);
 							initPromise = null;
+							if (me.config.serviceWorker && 'serviceWorker' in navigator) {
+								navigator.serviceWorker.register(me.config.serviceWorker);
+							}
 							resolve();
 						});
 					});