LibHTML: Add LayoutNode classes for "display: list-item" and its marker

This patch removes the hard-coded hack for "display: list-item" from
LayoutBlock and adds LayoutListItem and LayoutListItemMarker.

Elements with "display: list-item" now generate a LayoutListItem, which
may then also generate a LayoutListItemMarker if appropriate. :^)

Libraries/LibHTML/Layout/LayoutListItemMarker.h

@@ -0,0 +1,14 @@
+#pragma once
+
+#include <LibHTML/Layout/LayoutNode.h>
+
+class LayoutListItemMarker final : public LayoutNode {
+public:
+    LayoutListItemMarker();
+    virtual ~LayoutListItemMarker() override;
+
+    virtual void render(RenderingContext&) override;
+
+private:
+    virtual const char* class_name() const override { return "LayoutListItemMarker"; }
+};