Add QML language server configuration

This adds the new QML language server to the default configuration.

The qmlls binary is shipped as a binary with the qtdeclarative repo. Since it is only available in Qt6 and pretty much all distros compile qt-declarative6 such that all installed tools have 6 as suffix, it makes sense to add qmlls6 instead of qmlls by default.

Long term we might want to change it to qmlls, but it should be ok for now.