Add exit_river keybinding

Recently, river removed the hardcoded Ctrl+Alt Delete keybinding that
exits river and replaces it with a new `exit_session` request. This adds
support for that request via the new `exit_session` bind. We also added
3 hardcoded default keybinds to: exit river, reload the config, and
open foot. This way, if the config fails to load or is missing, you
should still be able to try reload. I guess you're still SOL if you have
at least one keybind and it's not reload_config, but you do what you can
do.

examples/config.kdl

@@ -83,6 +83,8 @@ keybinds {
     toggle_fullscreen Mod4 F
     // Close the currently-focused window
     close_window Mod4+Shift Q
+    // Exit the entire river session
+    exit_river Ctrl+Alt Delete
     // Move windows up or down the stack
     swap_next Mod4+Shift N
     swap_prev Mod4+Shift P