Make documentURI and URL not readonly (#13982)

These aren't actually readonly right now because `@readonly` means
nothing, and the setters are configured in php_dom.c. So no functional
changes here.

DOM spec marks these as readonly, but the problem is that this reduces
usefulness in XML contexts (like WSDL scheme handling). In context of a
browser, for which DOM was designed, this actually makes sense to have
as readonly because it is tied to the origin of the page etc. But PHP is
not a browser. This also wasn't readonly in "old DOM".

ext/dom/php_dom.stub.php

@@ -1515,9 +1515,7 @@ abstract class Document extends Node implements ParentNode
     {
         /** @readonly */
         public Implementation $implementation;
-        /** @readonly */
         public string $URL;
-        /** @readonly */
         public string $documentURI;
         public string $characterSet;
         public string $charset;