Create a single, hybrid Microsoft.ReactNative NuGet with both Paper and Fabric dlls

[jonthysell]

Summary

Put both fabric and paper dlls into the same dll, and use the targets to correctly set which one to be loaded by the project.

Motivation

To resolve the issue of having a "fabric" nuget package that relies on overloading the prerelease version tag.

Basic Example

No response

Open Questions

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