EventManager: add hasListener

Not sure if this should be in `EventTarget` or `EventManager`, here goes nothing.

src/browser/EventManager.zig

@@ -197,6 +197,21 @@ pub fn dispatchWithFunction(self: *EventManager, target: *EventTarget, event: *E
     try self.dispatchAll(list, target, event, &was_dispatched);
 }
 
+/// Returns true if there's at least 1 listener for given `event_type`.
+pub fn hasListener(self: *const EventManager, event_target: *const EventTarget, typ: []const u8) bool {
+    const list = self.lookup.get(@intFromPtr(event_target)) orelse return false;
+
+    var current_node = list.first;
+    while (current_node) |node| : (current_node = node.next) {
+        const listener: *const Listener = @alignCast(@fieldParentPtr("node", node));
+        if (listener.typ.eqlSlice(typ)) {
+            return true;
+        }
+    }
+
+    return false;
+}
+
 fn dispatchNode(self: *EventManager, target: *Node, event: *Event, was_handled: *bool) !void {
     const ShadowRoot = @import("webapi/ShadowRoot.zig");