Add custom function to generate asset manifest

[iansu]

Initial work on improving asset manifest. This moves the current output of all files under a files key. This is a breaking change so we want to do this as part of 3.0. This will allow us to add additional information to the manifest under other top-level keys in the future. See #5513.

[iansu]

This now generates something like this:

{
  "files": {
    "main.css": "/static/css/main.1a9471a3.chunk.css",
    "main.js": "/static/js/main.28647029.chunk.js",
    "main.js.map": "/static/js/main.28647029.chunk.js.map",
    "runtime~main.js": "/static/js/runtime~main.a8a9905a.js",
    "runtime~main.js.map": "/static/js/runtime~main.a8a9905a.js.map",
    "static/js/2.b41502e9.chunk.js": "/static/js/2.b41502e9.chunk.js",
    "static/js/2.b41502e9.chunk.js.map": "/static/js/2.b41502e9.chunk.js.map",
    "index.html": "/index.html",
    "precache-manifest.ba19183ceb6039c0e3ca4fd52fecfd0b.js": "/precache-manifest.ba19183ceb6039c0e3ca4fd52fecfd0b.js",
    "service-worker.js": "/service-worker.js",
    "static/css/main.1a9471a3.chunk.css.map": "/static/css/main.1a9471a3.chunk.css.map",
    "static/media/logo.svg": "/static/media/logo.5d5d9eef.svg"
  }
}

I named the new top level key files but I'm open to suggestions if anyone has a better idea.

[ianschmitz]

The paths are correct when using PUBLIC_URL I assume?

[iansu]

Yes. Building with PUBLIC_URL=http://example.com yarn build produces this output:

{
  "files": {
    "main.css": "http://example.com/static/css/main.1a9471a3.chunk.css",
    "main.js": "http://example.com/static/js/main.230a415d.chunk.js",
    "main.js.map": "http://example.com/static/js/main.230a415d.chunk.js.map",
    "runtime~main.js": "http://example.com/static/js/runtime~main.f7243f3d.js",
    "runtime~main.js.map": "http://example.com/static/js/runtime~main.f7243f3d.js.map",
    "static/js/2.b41502e9.chunk.js": "http://example.com/static/js/2.b41502e9.chunk.js",
    "static/js/2.b41502e9.chunk.js.map": "http://example.com/static/js/2.b41502e9.chunk.js.map",
    "index.html": "http://example.com/index.html",
    "precache-manifest.f3fb1479ad323e31d8e31c3e6e0b140e.js": "http://example.com/precache-manifest.f3fb1479ad323e31d8e31c3e6e0b140e.js",
    "service-worker.js": "http://example.com/service-worker.js",
    "static/css/main.1a9471a3.chunk.css.map": "http://example.com/static/css/main.1a9471a3.chunk.css.map",
    "static/media/logo.svg": "http://example.com/static/media/logo.5d5d9eef.svg"
  }
}