This commit implements solution configuration to project configuration mapping.
Previously, when a project was added to the solution with the `sln add`
command, solution configurations would be mapped to a project configuration and
platform of the same name, regardless of whether or not the project had a
configuration or platform of that name. This caused the solution to appear
dirty when opened in Visual Studio if the configuration or platform did not
exist at the project level because Visual Studio would attempt to correct the
mapping.
The fix is to check what configurations and platforms are supported by the
project and only map to what is present. If a solution configuration can't be
mapped, the first configuration/platform supported by the project is chosen;
this is consistent with how Visual Studio does the fallback mapping.
Fixes#6221.
