main: Destroy GjsContext before exit

main: Destroy GjsContext before exit

This will be required in the upcoming version of GJS.

The reference count on ShellGlobal is 2 at this point, because JS holds a
reference due to the "window.global = Shell.Global.get()" line in
ui/environment.js. Therefore, destroy the GjsContext first, then unref
the ShellGlobal object.

Cleaning up ShellGlobal was previously only enabled behind a debug
environment variable, but it should be required now.

https://bugzilla.gnome.org/show_bug.cgi?id=775374