LibWeb: Parse "display: flex" and create BlockBox layout nodes for them

I'm not 100% sure that BlockBox is the right layout node for flex
containers, but it's the most obviously fitting one we already have.

Userland/Libraries/LibWeb/CSS/StyleValue.h

@@ -90,6 +90,7 @@ enum class Display {
     TableColumn,
     TableColumnGroup,
     TableCaption,
+    Flex,
 };
 
 enum class WhiteSpace {