LibWeb: Start work towards modern CSS "display" values

Until now, we've internally thought of the CSS "display" property as a
single-value property. In practice, "display" is a much more complex
property that comes in a number of configurations.

The most interesting one is the two-part format that describes the
outside and inside behavior of a box. Switching our own internal
representation towards this model will allow for much cleaner
abstractions around layout and the various formatting contexts.

Note that we don't *parse* two-part "display" yet, this is only about
changing the internal representation of the property.

Spec: https://drafts.csswg.org/css-display

Userland/Libraries/LibWeb/CSS/StyleValue.h

@@ -22,6 +22,7 @@
 #include <AK/WeakPtr.h>
 #include <LibGfx/Bitmap.h>
 #include <LibGfx/Color.h>
+#include <LibWeb/CSS/Display.h>
 #include <LibWeb/CSS/Length.h>
 #include <LibWeb/CSS/Parser/StyleComponentValueRule.h>
 #include <LibWeb/CSS/PropertyID.h>
@@ -90,24 +91,6 @@ enum class Cursor {
     ZoomOut,
 };
 
-enum class Display {
-    None,
-    Block,
-    Inline,
-    InlineBlock,
-    ListItem,
-    Table,
-    TableRow,
-    TableCell,
-    TableHeaderGroup,
-    TableRowGroup,
-    TableFooterGroup,
-    TableColumn,
-    TableColumnGroup,
-    TableCaption,
-    Flex,
-};
-
 enum class FlexBasis {
     Content,
     Length,