Fix hexadecimal numbers always prefixed with 0x

If a number was just "42", it will be treated as 0x42. This commit fixes it so 0x hexadecimal prefixes are not emitted if they're explicitly omitted.

ILSourceParser/Parser.cs

@@ -521,7 +521,7 @@ internal static Parser<char> ParseHexChar()
     internal static Parser<string> ParseHexNumber()
     {
         return (
-                from prefix in ParseHexPrefix()
+                from prefix in Parse.String("0x")
                 from numbers in ParseHexChar().AtLeastOnce().Text()
                 select $"0x{numbers}"
             )