LibWeb: Add roman numerals as a list-style for ol's

This patch adds support for the identifiers upper-roman and lower-roman
of the list-style property.

Userland/Libraries/LibWeb/CSS/Identifiers.json

@@ -107,6 +107,7 @@
   "lowercase",
   "lower-alpha",
   "lower-latin",
+  "lower-roman",
   "medium",
   "move",
   "ne-resize",
@@ -163,6 +164,7 @@
   "uppercase",
   "upper-alpha",
   "upper-latin",
+  "upper-roman",
   "visible",
   "vertical-text",
   "wait",

Userland/Libraries/LibWeb/CSS/StyleProperties.cpp

@@ -645,6 +645,10 @@ Optional<CSS::ListStyleType> StyleProperties::list_style_type() const
         return CSS::ListStyleType::UpperAlpha;
     case CSS::ValueID::UpperLatin:
         return CSS::ListStyleType::UpperLatin;
+    case CSS::ValueID::UpperRoman:
+        return CSS::ListStyleType::UpperRoman;
+    case CSS::ValueID::LowerRoman:
+        return CSS::ListStyleType::LowerRoman;
     default:
         return {};
     }

Userland/Libraries/LibWeb/CSS/StyleValue.h

@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
+ * Copyright (c) 2021, Tobias Christiansen <tobi@tobyase.de>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -173,8 +174,10 @@ enum class ListStyleType {
     DecimalLeadingZero,
     LowerAlpha,
     LowerLatin,
+    LowerRoman,
     UpperAlpha,
     UpperLatin,
+    UpperRoman,
 };
 
 enum class Overflow : u8 {