gjs: clean up

* Do not propagate spidermonkey, it is only needed gjs to link against.
* Split out dev output, any propagated dependencies should go there.
* Drop unnecessary sed, the readline thing is no longer there.
* Add passthru.tests to allow running installed tests easily.
* Drop pango from buildInputs, we only need it as a transitive dependency of gtk3 for installed tests; gjs actually depends on cairo.
* Move libxml2 to nativeBuildInputs, it is called by glib-compile-resources when xml-stripblanks preprocessing is used.
* Propagate glib, as it is in Requires in the pc file.

It is just 2.5 MB closure reduction but every megabyte counts.