Setting to change the entry point of the packages of a site [8pt]

[SantosGuillamot]

This is needed for AMP feature. For full context please check out the final Implementation proposal.

---

Some packages may require a different entry point for AMP because sometimes they are going to contain code that is not needed in the React version and we need to provide tools to make the bundle size of the React version as slim as possible.

This means that instead of using src/index.js, packages can use src/amp.js or src/amp/index.js as entry points when the site is an AMP site. This is of course optional, and the packages that don't need it or don't implement this can still use src/index.js.

It will be managed by a new site setting on the frontity.settings.js file:

export default {
  name: "my-amp-site",
  entryPoints: "amp",
  state: {
    //...
  },
  packages: [
    //...
  ],
};