Add support for multiple entrypoints

[tatarianBarbarian]

Now, the code works with a single entry point.

It's possible for code to have multiple of them, and we should check how to handle it the right way.
We may want to split navigation to avoid possible confusion, for example.

This task is for checking what can go wrong and fixing it.

• Adapt navigation - Edit navigation format #36
• Make sure cross-module/cross-project links work - Fix generated folder structure #37
• Fix folder names for Astro to make pages work - Fix cross project reference links #38

[tatarianBarbarian]

Navigation object should be changed if we want to use it with multiple entry points.

We will add a new field to the navigation object, type, which can be flat | modular.

For flat navigation there will be following structure:

{
  type: 'flat',
  items: [
    { title: '...', url: '...' }
  ]
}

For modular navigation structure will be following:

{
  type: 'modular',
  modules: [
    {
      moduleName: 'module1',
      items: [{ title: '...', url: '...' }]
    },
    {
      moduleName: 'module2',
      items: [{ title: '...', url: '...' }]
    }
  ]
}

[ai]

I hope in Sidebar component we will be able to make this navigation flat.

For Logux we need to mix all components, but for Nano Stores it will be better to keep them separately.