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/StyleValue.h

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