Introduce pagesManifest

[Janpot]

Working towards reducing the role of the appDom to just a single page. The end-goal is to fully isolate low-code page building from the pro-code page building. That enables us to unbundle the editor and develop Toolpad low-code abilities independently from pro-code abilities.

• The pages manifest will be used to do routing/navigation in the app, for this it will be possible to import it as 'virtual:toolpad-files:pages-manifest.json'.
• It will also be available to the builder so it can generate components for the code pages
• We will also make it available to the editor through an API so it can render the pages hierarchy with it
• We already have something similar for components that is used to import them in the runtime
• We will use this components manifest in the editor to populate the components hierarchy Remove components from appDom #3017
• We will build a similar thing for the theme