Callers can pass an exit code to nsIAppStartup::Quit and it will be returned from the process when
it exits.
Note that I have using uint16_t as the exit code because on Windows the exit code can be a uint and
elsewhere it is an int. A uint16_t will safely convert to either of those and no-one will ever need
more than 64k exit codes!
Differential Revision: https://phabricator.services.mozilla.com/D96857
