VS: Add COMMON_LANGUAGE_RUNTIME support for "netcore"

I've run into the problem of being unable to use the netcore CLR for managed/mixed projects and upon some research I've found #22054 (closed) and realized it's been over a year since it has been reported, so here's my attempt on fixing it.

When specifying netcore for COMMON_LANGUAGE_RUNTIME it will now add the <CLRSupport> tags to the project. I'm not entirely sure this is the correct way on going about this since I had to slightly modify some logic as previously having any value for COMMON_LANGUAGE_RUNTIME meant its managed where now having netcore is still mixed.

Fixes: #22054 (closed)
Topic-rename: vs-CLRSupport