VS: Teach include_external_msproject about non-C++ projects (#14661)

Teach CMake to guess the project type guid based on the project file
extension.  This allows non-C++ projects like *.vbproj or *.csproj
to be included.
Merge topic 'vs-external-project-type' into next

a79cbdc VS: Teach include_external_msproject about non-C++ projects (#14661)