cmake file api

[rwols]

• [x] trim fat from this plugin
• [x] use the new file api, requires cmake 3.15
• [x] modernize vcvarsall.bat handling for windows, deprecate vs2013 and older
• [x] allow any "Visual Studio x yyyy" generator string
• [x] account for platform for generators
• [x] account for toolset for generators
• [x] make a migration path for old settings, e.g. target_architecture --> platform, visual_studio_versions --> vs_major_version