add script for forced-wayland firefox

you can set this e.g. as xdg-open target to correctly invoke wayland-
enabled firefox for programs which need `unset GDK_BACKEND`

dotfiles/bin/.bin/firefox

@@ -0,0 +1,6 @@
+#!/bin/sh
+echo $@
+
+real_firefox=$(which --skip-tilde firefox)
+
+GDK_BACKEND=wayland "${real_firefox}" "$@"