Add option to build as shared lib

[bmarques1995]

Uses BUILD_SHARED_LIBS standard CMake option to build as a DLL/so vs. a static C++ library.

[walbourn]

Thanks for the contribution. I have some follow-up changes here to add VERSIONINFO to the produced DLL, and a few minor whitespace changes. I also want to make sure there's coverage for this scenario so I'll add jobs to some GitHub Actions to make sure this works long-term.

I also should look at DirectXMesh and UVAtlas for the same scenario. It still doesn't make any sense to me to add this to DirectX Tool Kit at this time.