Don't include source map files in service worker output

[maxmilton]

When generating the service worker manifest file (app/manifest/service-worker.js) it currently also includes source map files (e.g. "client/fbd4e5d87f8d7dec1966/main.0.js.map"). These files don't belong here and should be excluded. Perhaps there could be some way for the developer to exclude arbitrary files here?

Once this is implemented, it would be worth considering enabling source map generation in production builds.