add an XpcConnection.prototype.stop() method

[chee]

for #16 and @sandeepmistry/noble#299 and this comment
on @sandeepmistry/noble#577.

now admittedly i have no idea what i'm doing here, but it does appear to work. at first i tried canceling the connection with xpc_connection_cancel(this->xpcConnection) but it sure took that badly

so now it is closing and deleting the handle like it would in the destructor

i do not know if this is the right thing to do, but it seems to work (does not prevent the node process from ending)