Add official support for pdf2.0 (#125)

Author: PrinsFrank

src/Document/Version/Version.php

@@ -19,6 +19,7 @@ enum Version: string implements NameValue {
     case V1_5 = '1.5';
     case V1_6 = '1.6';
     case V1_7 = '1.7';
+    case V2_0 = '2.0';
 
     public static function length(): int {
         return 3;