feat(event): introduce .bell system event

src/event.zig

@@ -13,6 +13,9 @@ pub const SystemEvent = union(enum) {
     /// Usually no `Container` nor `Element` should act on that event, as it
     /// only serves for event based loops to force a re-draw with a new `Event`.
     resize,
+    /// Ring the terminal bell to notify the user. This `Event` is handled by
+    /// every `Container` and will not be passed through the container tree.
+    bell,
     /// Error event to notify other containers about a recoverable error
     err: struct {
         /// actual error