[Translator] Mark $message and $messageLength as readonly

Author: Kocal

src/Translator/src/Intl/IntlMessageParser.php

@@ -22,9 +22,9 @@
  */
 class IntlMessageParser
 {
-    private AbstractString $message;
+    private readonly AbstractString $message;
     // Minor optimization, this avoid a lot of calls to `$this->message->length()`
-    private int $messageLength;
+    private readonly int $messageLength;
 
     private Position $position;
     private bool $ignoreTag;