Enable loading the bundle from an embedded resource

[asklar]

We should enable embedded resources as a mechanism to carry the JS bundle with the app; this will be relevant in the win32/WinAppSDK world if we decide to / for customers that don't want to deal with MRT/MRTCore.

One way to do this is to reuse the existing BundleRootPath, which currently accepts file paths as well as URIs. For appx, we accept ms-appx etc. We could start accepting something like resource:// as a uri scheme.

The desktop DLL used by Office already has code for reading from an embedded resource; once we add this functionality to the official library, we can work with Office to move to the standard API.

This will accrue towards #8116