LibWeb: Assign a unique ID to each HTML task

And return that ID when queueing an element task. This is required for
some task tracking - specifically, the HTML ToggleTaskTracker struct.

Userland/Libraries/LibWeb/HTML/EventLoop/Task.h

@@ -59,6 +59,7 @@ public:
     }
     ~Task();
 
+    int id() const { return m_id; }
     Source source() const { return m_source; }
     void execute();
 
@@ -69,6 +70,7 @@ public:
 private:
     Task(Source, DOM::Document const*, JS::SafeFunction<void()> steps);
 
+    int m_id { 0 };
     Source m_source { Source::Unspecified };
     JS::SafeFunction<void()> m_steps;
     JS::Handle<DOM::Document const> m_document;