Avoid a race between reading from and writing to the `importScripts` array

[errendir]

When WorkboxPlugin.GenerateSW is called it reads from the importScripts array. Webpack instance called from build-custom-worker.js appends to that array via the success callback. There is no guarantee that webpack compilation will finish before GenerateSW is called. This results in a bug where sometimes no scripts will be imported in the Service Worker code.

From my experiments to replicate this issue you need to ensure the webpack compilation is delayed enough. Then GenerateSW will not pick up any importScripts. For an example project triggering this bug see: https://github.com/errendir/next-pwa/tree/import-scripts-race-demo/examples/custom-worker

After running yarn build, please open the public/sw.js and format it with Prettier. You will find that no scripts have been imported into the ServiceWorker generated by workbox:

define(["./workbox-6316bd60"], function (e) {
  "use strict";
  importScripts(),
    self.skipWaiting(),
    e.clientsClaim(),

The above example works by feeding a very large function into Webpack during custom worker compilation. This slows it down enough so that the race condition can be highlighted.

This PR fixes this issue by appending the name of the custom worker script to the importScripts synchronously before GenerateSW is called. Note that this may be before the file is generated, but AFAIK GenerateSW doesn't verify if the script exists so the problem is solved