LibHTML: Add stub classes for basic table layout

This class introduces LayoutTable, LayoutTableRow and LayoutTableCell.
These are produced by "display" values table, table-row and table-cell
respectively.

Note that there's no layout happening yet, I'm just adding the classes.

Libraries/LibHTML/Layout/LayoutTableRow.h

@@ -0,0 +1,14 @@
+#pragma once
+
+#include <LibHTML/Layout/LayoutBox.h>
+
+class LayoutTableRow final : public LayoutBox {
+public:
+    LayoutTableRow(const Element&, NonnullRefPtr<StyleProperties>);
+    virtual ~LayoutTableRow() override;
+
+    virtual void layout() override;
+
+private:
+    virtual const char* class_name() const override { return "LayoutTableRow"; }
+};