Support resource:/// URL's in GNOME_SHELL_JS envvar

It can be useful to augment the shell's search path by doing

 GNOME_SHELL_JS=resource:///org/gnome/shell:<mypath>

But this doesn't work because resource: is split off. Special
case path elements that are just 'resource' and recombine
them with the next element.

https://bugzilla.gnome.org/show_bug.cgi?id=730409