Provide an opt-out define for std::format support

As discussed on QtCS, the users should not need to include additional headers to enable std::format support for a certain Qt type.

The conclusion is that we add all the relevant code directly in the class header, but guard it with an opt-out define, which allows to disable std::format support for those who are not interested in it.

Note that it should be a #define, not a QT_FEATURE, because the user's configuration might be different from what was used to build Qt itself.