Using multiple nested contexts on the same thread isn't normal usage of
Spidermonkey and triggers bugs in Spidermonkey that might not be caught
in upstream testing. (It is theoretically supported, to at least some extent.)

When calling a callback, the best context to use is the current context.
If that's not available, then using the default context from the GjsContext
is usually sensible.

Add functions:
 gjs_runtime_push_context()
 gjs_runtime_pop_context()
 gjs_runtime_get_current_context()

One potential concern here is the extensive use of GDataset to get data
from a context, which involves locking overhead and hash table lookups and
can be slow; since we can't use GJS with multiple JS_Runtimes in the same
process due to the limitations of toggle references, perhaps we should enforce
that and use global variables.

https://bugzilla.gnome.org/show_bug.cgi?id=622896