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/LayoutTableCell.h

@@ -0,0 +1,12 @@
+#pragma once
+
+#include <LibHTML/Layout/LayoutBlock.h>
+
+class LayoutTableCell final : public LayoutBlock {
+public:
+    LayoutTableCell(const Element&, NonnullRefPtr<StyleProperties>);
+    virtual ~LayoutTableCell() override;
+
+private:
+    virtual const char* class_name() const override { return "LayoutTableCell"; }
+};