Add support for WIN32 and MACOSX_BUNDLE to qt6_add_executable()

The standard CMake add_executable() command accepts the keywords WIN32 and MACOSX_BUNDLE. When building for the relevant platform, these keywords set the corresponding target property, otherwise the keyword is ignored. It would be good if qt6_add_executable() also understood these keywords so that its behavior is more compatible with the add_executable() command. This would allow code to be expressed more simply. Here's the before/after comparison:

Before:

qt_add_executable(basic
    main.cpp
)
set_target_properties(basic PROPERTIES
    WIN32_EXECUTABLE TRUE
    MACOSX_BUNDLE TRUE
)

After:

qt_add_executable(basic WIN32 MACOSX_BUNDLE
    main.cpp
)

 
For the Android case, the implementation of qt6_add_executable() would have to explicitly check for and silently drop these keywords, since it creates a library rather than an executable for the Android case.