Correct "fire_exit" event back to "fish_exit"

This was causing fish_exit to not fire, which caused (among other things)
leaking tmux processes from the tests.

This was bisected to eacbd6156d

src/bin/fish.rs

@@ -763,7 +763,7 @@ fn main() {
     // Trigger any exit handlers.
     event::fire_generic(
         parser,
-        L!("fire_exit").to_owned(),
+        L!("fish_exit").to_owned(),
         vec![exit_status.to_wstring()],
     );
 

tests/checks/fish_exit.fish

@@ -0,0 +1,8 @@
+#RUN: %fish -C 'set -g fish %fish' %s
+
+# fish_exit fires successfully.
+echo 'function do_exit --on-event fish_exit; echo "fish_exiting $fish_pid"; end' > /tmp/test_exit.fish
+$fish /tmp/test_exit.fish
+# CHECK: fish_exiting {{[0-9]+}}
+
+rm /tmp/test_exit.fish