Running the text templating engine within Visual Studio for Mac fails
since the RuntimeInfo fails to find any SDK directory.

When using the Roslyn code compiler there is now no exception if no
SDK is found. The SDK location is not needed for the Roslyn compiler
since there is no external csc.dll to run.

Also use a better default C# lang version for the Roslyn code compiler
instead of 7.0

The CSharpParseOptions was not using the best language version. The
CSharpParseOptions was created but no language version was used from
the RuntimeInfo nor the language version specified by the Roslyn code
compiler.

Not needed on the RuntimeInfo class anymore.