feat(app): signal WINCH for .resize system event

This allows the application to automatically re-draw and resize if the
application receives the signal by the terminal emulator.

src/event.zig

@@ -8,6 +8,8 @@ pub const SystemEvent = union(enum) {
     init,
     /// Quit event to signify the end of the event loop (rendering should stop afterwards)
     quit,
+    /// Resize event to signify that the application should re-draw to resize
+    resize,
     /// Error event to notify other containers about a recoverable error
     err: struct {
         /// actual error