[feature request] check if systemd-resolved in installed

[th1031]

Installed the script on Debian 11 which I had converted to use systemd-resolved so as to support systemd-containerd. The installation left me without internet connectivity due to your forced installation of resolvconf. I was able to restore connectivity after removing the resovlconf package and re-creating the systemd-resolved symlink from /etc/resolv.conf to /run/systemd/resolve/stub-resolv.conf

I don't have any problem with wireguard when using systemd-resolved so checking if it's installed before installing resolvconf may save others some issues. Note the systems-resolved debian package provides a symlink from /sbin/resolvconf to /usr/bin/resolvectl.