nixos/plasma5: Enable browser integration

This option is not documented anywhere and while it may be set
in configuration.nix to enable integration, having it on by
default when using both plasma and firefox is a great convenience;
just like all other desktop environments do it already.

nixos/modules/services/x11/desktop-managers/plasma5.nix

···
 
       # Update the start menu for each user that is currently logged in
       system.userActivationScripts.plasmaSetup = activationScript;
+
+      nixpkgs.config.firefox.enablePlasmaBrowserIntegration = true;
     })
   ];