WIP: upgrade to Workbox v6 and TypeScript service worker

There are many things broken right now, so don't trust this.

References:
<https://stackoverflow.com/questions/56356655/structuring-a-typescript-project-with-workers>
<GoogleChrome/workbox#2642>
<https://gist.github.com/dsebastien/12c47fdb6517cfdab9473297f4472d22>

src/scripts/tsconfig.json

@@ -0,0 +1,6 @@
+{
+ "extends": "../generic-tsconfig.json",
+ "references": [
+ {"path": "../service-worker"}
+ ]
+}

src/scripts/service-worker.js → src/service-worker/service-worker.ts

@@ -1,20 +1,25 @@
-importScripts('https://storage.googleapis.com/workbox-cdn/releases/5.1.2/workbox-sw.js');
+import { CacheableResponsePlugin } from 'workbox-cacheable-response';
+import { ExpirationPlugin } from 'workbox-expiration';
+import { cleanupOutdatedCaches, precacheAndRoute } from 'workbox-precaching';
+import { registerRoute } from 'workbox-routing';
+import { CacheFirst } from 'workbox-strategies';
 
 // The following call will be populated automatically with the precached file
 // data during the build step
-workbox.precaching.precacheAndRoute(self.__WB_MANIFEST);
+cleanupOutdatedCaches();
+precacheAndRoute(self.__WB_MANIFEST);
 
 // Cache Google fonts
-workbox.routing.registerRoute(new RegExp('^https://fonts.(?:googleapis|gstatic).com/(.*)'), new workbox.strategies.CacheFirst({
+registerRoute(new RegExp('^https://fonts.(?:googleapis|gstatic).com/(.*)'), new CacheFirst({
  cacheName: 'google-fonts',
  plugins: [
  // When the cap is reached, the oldest entries are purged
- new workbox.expiration.ExpirationPlugin({
+ new ExpirationPlugin({
  maxEntries: 30
  }),
  // The Google Fonts CSS response is an opaque (non-CORS) response with a
  // status code of 0, so we need to enable caching for that type of response
- new workbox.cacheableResponse.CacheableResponsePlugin({
+ new CacheableResponsePlugin({
  statuses: [0, 200]
  })
  ]

src/service-worker/tsconfig.json

@@ -0,0 +1,7 @@
+{
+ "extends": "../generic-tsconfig.json",
+ "compilerOptions": {
+ "lib": ["esnext", "webworker"],
+ "moduleResolution": "node"
+ }
+}