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/CSS/Default.css

@@ -109,3 +109,15 @@ hr {
 blink {
     display: inline;
 }
+
+table {
+    display: table;
+}
+
+tr {
+    display: table-row;
+}
+
+td {
+    display: table-cell;
+}