Starting with mozjs31, JS_Init() is required. Calling JS_ShutDown() on
exit is not required, but may become so in the future.

This does so in the constructor and destructor of a static object.
Normally this is discouraged because the order in which the constructors
and destructors are called is not guaranteed, but I don't think that is a
problem here since it's unlikely that anyone will try to use GJS API from
a static constructor.

However, API clients still must unref any GjsContext before the program
shuts down. Usually this is not a problem, unless a JS script calls
System.exit(), in which case the code would exit immediately without
unreffing the GjsContext before JS_ShutDown was called. Instead, we make
System.exit() throw an uncatchable exception, which is propagated all the
way up to JS::Evaluate() and turned into a GError with code
GJS_ERROR_SYSTEM_EXIT, thrown from gjs_context_eval().

For this, we need to expose GjsError and its error codes in the public
API. (They should have been exposed already, because gjs_context_eval()
could already throw GJS_ERROR_FAILED.)

Finally, the tests added as part of this patch made the testSystemExit
script obsolete.

https://bugzilla.gnome.org/show_bug.cgi?id=751252