Generate html-manifest.json of html assets.

[patope]

It's non-trivial to reconstruct html page using asset-manifest.json currently. See: #5513

This change adds alternative asset file for that purpose. ExtractHtmlManifestPlugin extracts html page assets as defined here https://github.com/jantimon/html-webpack-plugin/blob/master/lib/hooks.js#L21 to separate file html-manifest.json. This file can be used to reconstruct html page.

example of generate html-manifest.json:

{
  "publicPath": "/",
  "js": [
    "/static/js/runtime~main.229c360f.js",
    "/static/js/1.82dafdb5.chunk.js",
    "/static/js/main.c9b66095.chunk.js"
  ],
  "css": [
    "/static/css/main.90a4e9dc.chunk.css"
  ]
}

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[stale]

This pull request has been automatically marked as stale because it has not had any recent activity. It will be closed in 5 days if no further activity occurs.

[stale]

This pull request has been automatically closed because it has not had any recent activity. If you have a question or comment, please open a new issue. Thank you for your contribution!

[bartlangelaan]

Shouldn't be stale.

Something like this is really useful for people that want to integrate CRA in another application.

Please open again, or provide feedback 😁

[evang522]

We are also needing this in order to construct an HTML file dynamically outside of the react environment. This is a huge step towards making CRA compatible with multiple different environments. Please reopen!