VS: Add support for Visual Studio 2019 Preview 2
Preview 2 seems to have killed the new generator. It can't find the VS install. Looks like they changed a filename:
C:/Program Files (x86)/Microsoft Visual Studio/2019/Preview/VC/Auxiliary/Build/Microsoft.VCToolsVersion.default.txt
C:/Program Files (x86)/Microsoft Visual Studio/2019/Preview/VC/Auxiliary/Build/Microsoft.VCToolsVersion.v142.default.txt
And as commented in the source, the toolset is now v142.