Allow packages to modify the settings of a site [13pt]

[SantosGuillamot]

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

---

The @frontity/amp package needs to be able to change the entryPoints and bundles settings.

This will be managed by a settings export in the frontity.config.js file of this package.

// packages/amp/frontity.config.js
export const settings = ({ settings }) => {
  settings.entryPoints = "amp";
  settings.bundles = ["server"];
};

The settings export of the frontity.config.js file of each package will be executed right after getting the settings from the frontity.settings.js file.