restructure repository and support UPM

[JoC0de]

restructure repository
add support for UPM

[JoC0de]

@jwittner can you have a short look at the new repository structure (just look at the resulting https://github.com/JoC0de/NuGetForUnity/tree/feature/update-project-layout)?
It is not required to change the repository structure to support UPM, I just wanted to "clean up" the structure because when we support UPM we can't change it (at least the part where the package.json is located).
For easier understanding:

• src/NuGetForUnity/ is the UPM package, it contains only the code that is needed by the "user"
• src/NuGetForUnity.Tests/ is the Unity project that contains the tests, it includes NuGetForUnity as a local package.

You can ignore that the GitHub actions on the PR are failing this is just "by design" they are working on https://github.com/JoC0de/NuGetForUnity/ and will work as soon as the PR is merged.

[jwittner]

Looks reasonable!

[JoC0de]

@jwittner Ok thanks for the feedback