[BUG] Empty object when adding runtimeCaching custom plugins with workbox-webpack-plugin

[tsirlucas]

Library Affected:
workbox-webpack-plugin and workbox-build

Browser & Platform:
not browser-related

Issue or Feature Request Description:
When adding custom plugins to runtimeCaching option on workbox-webpack-plugin, the output sw comes with an empty object instead of the object with functions to be executed.

I think this is happening because of this line once JSON.stringify removes functions. I've managed to fix the issue on a fork with this commit parsing the object to string instead (I think it's the desired behavior and it works).

My solution doesn't parse comments and once the populate-sw-template puts the plugin content inline, single-line comments can break everything. I can work on a solution for this either but I'd like to know if you guys would accept a PR and what would be the desired behavior when dealing with comments. Should them be removed or transformed into to multi-line comments?

Code to reproduce:

new GenerateSW({
    swDest: 'sw.js',
    clientsClaim: true,
    skipWaiting: true,
    importWorkboxFrom: 'local',
    navigateFallback: '/',
    runtimeCaching: [
      {
        urlPattern: /^https:\/\/api-random\.com\/random\//,
        handler: 'cacheFirst',
        options: {
          cacheName: 'random-cache',
          expiration: {
            maxEntries: 40,
            maxAgeSeconds: 200000,
          },
          cacheableResponse: {statuses: [0, 200, 201, 206, 301, 304, 302]},
          plugins: [{
            cacheWillUpdate: ({request, response}) => {
              return caches.match(request.url, {ignoreSearch: true}).then((res) => {
                /* ignore repeated songs */
                if (res) {
                  return null;
                }
          
                /* cache only save=true param */
                if (request.url.includes('?save=true')) {
                  return response;
                }
          
                /* ignore any other thing */
                return null;
              });
            },
            cachedResponseWillBeUsed: ({request, cachedResponse}) => {
              if (cachedResponse) {
                return cachedResponse;
              }
              /* this will match same url/diff query string where the original failed */
              return caches.match(request.url, {ignoreSearch: true});
            },
          }],
        },
      },
    ],
  });