This PR adds TargetPlatform.macOS to the TargetPlatform enum. This allows us to begin implementation of some adaptive UI based on which target platform is desired.
I haven't updated the tests here, that will come in a follow-up PR.
I updated the build files for manual_tests, as well as adding in a macos and web directory to allow the manual tests to be run on the web or on desktop.
The main change here are the parts that I added to the files in manual_tests/lib/... (the addition of kIsWeb to "if (!kIsWeb && Platform.isMacOS) {") The rest is just an update of the auto-generated code from flutter create.
