This gives us g_autoptr support for GjsContext.

It's appropriate to use G_DECLARE_FINAL_TYPE here even though the header
file previously defined GJS_CONTEXT_CLASS etc. macros which are only
defined by G_DECLARE_DERIVABLE_TYPE. This is because `struct
_GjsContextClass` was never exposed in the header file. Therefore,
defining a type that inherits from GjsContext was already impossible. We
can keep the GJS_CONTEXT_CLASS macros in the header for backwards
compatibility, though.