Standalone qmlls binary should support 6.8 but have newest features

Currently, building qmlls from dev does not really give a very usable qmlls binary for 6.8 projects, for example because qmltypesregistrar changed and therefore lots of warnings are emitted when .qmltypes files are read.

This means users using qmlls from dev on 6.8 projects will get unfixable warnings, which is not too nice. Try to experiment with a 6.8 version of qmlls that is patched to have the newest qmlls features but uses qt 6.8 otherwise (for example for the QtQmlCompiler Library).

If that gives a qmlls that works well on 6.8, try to build it as part of the GHA workflows.