Evaluate C++11 support for regular expressions

It is time to finally kill QRegExp.

It has been deprecated through the whole lifetime of Qt5 but cannot be removed because we use it in various places that are used by the bootstrap library – which can't use QRegularExpression because that requires libpcre, which has to be found by the configuration system before we can bootstrap. (It's possible the cmake port will remove this obstacle.)

One possible way out of this would be to use C+'s support for regular expressions (apparently present at C11, possibly enhanced since; we can use anything up to C+17 in Qt 6).
This subtask is here to record the results of investigating that possibility.