events: create an EventHandlerData struct

It simplifies the EventHandlerFunc creation and allows to insert user's
data.

src/dom/event_target.zig

@@ -73,9 +73,9 @@ pub const EventTarget = struct {
             self,
             alloc,
             eventType,
-            cbk,
-            capture orelse false,
             EventHandler,
+            .{ .cbk = cbk },
+            capture orelse false,
         );
     }