Support for package priorities [8pt]

[SantosGuillamot]

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

---

In some cases, we will need to defined packages priorities to define in which order they are executed.

For example, the @frontity/amp package needs to be executed before any other package because it is going to change the entryPoints setting and that will affect how the rest of the packages are imported.

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

// packages/amp/frontity.config.js
export const priority = 2;

Users can override this setting in the frontity.settings.js file:

export default {
  name: "my-amp-site",
  packages: [
    {
      name: "some-package",
      priority: 5, // Override default priority for this package.
      state: {
        // ...
      },
    },
  ],
};