allow expr-level exports

[sam0x17]

Right now #[docify::export] is only recognized when it is applied to Item, ImplItem, and a few others.

This would open it up to be able to be applied to anything in expr position if inside of an outer macro pattern on the parent item (attributes can't appear in expr position yet unless you are using outer macro pattern).

If we can get this in, it will unblock #10