Previously <gjs/gjs.h> pulled in a lot of stuff, and in particular,
<gjs/jsapi-util.h>, which in turn required <jsapi.h>.  For a simple
app that wants to embed GJS we should not be pulling that in.

So <gjs/gjs.h> is now the "simple" API that actually just includes
<gjs/context.h>, suitable for creating a context and calling eval().

<gjs/gjs-module.h> is now equivalent to the old <gjs/gjs.h>, it
pulls in the world.

Also, create a corresponding .pc file, gjs-internals-1.0.pc.  This one
includes mozjs as Requires, and adds the requisite Cflags.  For
gjs-1.0.pc, change the Requires to simply be gobject-2.0.
Conceptually, a gjs-devel RPM should not Require
gobject-introspection-devel or xulrunner-devel, and a simple embedder
program just using gjs_context_new()/gjs_context_eval() should not
have DT_NEEDED on gobject-introspection-1.0.so or mozjs.so.