LibWeb: Add list-style-type: decimal-leading-zero support

This doesn't exactly do what you would think from its name: It surely
adds an extra leading zero to the front of a number, but only if the
number is less than 10. CSS is weird sometimes.

Userland/Libraries/LibWeb/CSS/Identifiers.json

@@ -77,6 +77,7 @@
   "crosshair",
   "dashed",
   "decimal",
+  "decimal-leading-zero",
   "default",
   "disc",
   "dotted",

Userland/Libraries/LibWeb/CSS/StyleProperties.cpp

@@ -576,6 +576,8 @@ Optional<CSS::ListStyleType> StyleProperties::list_style_type() const
         return CSS::ListStyleType::Square;
     case CSS::ValueID::Decimal:
         return CSS::ListStyleType::Decimal;
+    case CSS::ValueID::DecimalLeadingZero:
+        return CSS::ListStyleType::DecimalLeadingZero;
     default:
         return {};
     }

Userland/Libraries/LibWeb/CSS/StyleValue.h

@@ -159,6 +159,7 @@ enum class ListStyleType {
     Circle,
     Square,
     Decimal,
+    DecimalLeadingZero,
 };
 
 enum class Overflow : u8 {