It's not possible to stop tracing an object in the middle of GC. However,
by using JS::ExposeObjectToActiveJS(), it is possible to mark an object
as reachable for the duration of one GC. This is exactly what we need for
closures, to keep the closure's callable object from disappearing while
GC is going on.

This requires adding a method, prevent_collection(), to GjsMaybeOwned.
This method is only valid in non-rooted mode. It also requires a bit of
GC API magic to avoid running afoul of some debug-mode assertions.
SpiderMonkey master at least has an official API for one of these bits of
magic, which we can switch to in SpiderMonkey 59.

https://bugzilla.gnome.org/show_bug.cgi?id=786668