LibWeb: Implement most of the CSS table fixup algorithm

This patch implements the "remove irrelevant boxes" and "generate
missing child wrappers" parts of table fixup.

"Generate missing parents" is left as a task for our future selves.

Libraries/LibWeb/Layout/TableCellBox.h

@@ -41,6 +41,8 @@ public:
 
     size_t colspan() const;
 
+    static CSS::Display static_display() { return CSS::Display::TableCell; }
+
 private:
     virtual float width_of_logical_containing_block() const override;
 };