nixos/picom: add experimentalBackends option
This option is only available as a command-line flag and not from the
config file, that is services.picom.settings. Therefore it is more
important that it gets its own option.
One reason one might need this set is that blur methods other than
kernel do not work with the old backends, see yshui/picom#464.
For reference, the home-manager picom module exposes this option too.