Clean up & add createOrbitApp

[dolanske]

This PR shuffles a few files around for better readability and also adds a createOrbitApp core method, which automatically registers vue router and offloads the main app creation from apps to the core package instead.

The idea is that the user needs to do only 3 steps in order to spin up the Orbit front end

1. Import the createOrbitApp
2. Create the proper platform controller (eg.: createWebPlatform)
3. Use the <OrbitApp /> component in the target application entrypoint

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I removed a few comments from platform/types.ts as they seemingly randomly documented some object keys but left most of them out. And I think the top-level comment should probably be rephrased and instead be put to README

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Additionally added a new depot package which servers as a layer between the backend and the ui app

[zealsprince]

I was told this would just work and I don't have to review it fully.