Add support for Qt WebEngine dictionaries

This change implements what was discussed at https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1020387.

It adds a file to /etc/environment.d that sets the QTWEBENGINE_DICTIONARIES_PATH environment variable so that Qt WebEngine can locate the dictionaries in /usr/share/hunspell-bdic/.

It also provides the convert-dict virtual package from qt6-webengine-dev-tools, and provides a symlinks from usr/bin/convert_dict to qwebengine_convert_dict so that Hunspell dictionaries can build-depend on it them in such a way they don't have to modify their code when Qt switches version in the future.