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/Identifiers.json

@@ -70,6 +70,7 @@
   "dotted",
   "double",
   "fixed",
+  "flex",
   "full-size-kana",
   "full-width",
   "groove",

Userland/Libraries/LibWeb/CSS/StyleProperties.cpp

@@ -410,6 +410,8 @@ CSS::Display StyleProperties::display() const
         return CSS::Display::TableHeaderGroup;
     case CSS::ValueID::TableFooterGroup:
         return CSS::Display::TableFooterGroup;
+    case CSS::ValueID::Flex:
+        return CSS::Display::Flex;
     default:
         return CSS::Display::Block;
     }

Userland/Libraries/LibWeb/CSS/StyleValue.h

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