mod(element): introduce deinit interface for deinitializing Elements automatically with the Container

examples/demo.zig

@@ -66,7 +66,6 @@ pub fn main() !void {
     try box.append(try App.Container.init(allocator, .{
         .rectangle = .{ .fill = .lightgreen },
     }, .{}));
-    defer box.deinit();
 
     var scrollable: App.Scrollable = .init(box, .disabled);