gi: object: add function to check if a JSObject contains a GObject
gi: function: add gjs_invoke_constructor_from_c

The purpose of this function is to be called by from C code without
creating an intermediate JSObject which we don't need because we're
already calling from the constructor of the object being created.

https://bugzilla.gnome.org/show_bug.cgi?id=621716
gi: add fundamental type support

This work in largely based on previous work from Johan Dahlin.

https://bugzilla.gnome.org/show_bug.cgi?id=621716