Allow by-ref assign to WeakMap even if object is not yet in the map (#20097)

Previously this failed as the read_dimension which is invoked by
ref-assign does not contain the logic to add the key, so it was required
to first write the value using a normal assignment and then thereafter
use the reference assignment.
This solves it by adding the necessary logic to assign references
directly.

UPGRADING

@@ -23,6 +23,10 @@ PHP 8.6 UPGRADE NOTES
 2. New Features
 ========================================
 
+- Core:
+  . It is now possible to use reference assign on WeakMap without the key
+    needing to be present beforehand.
+
 - Intl:
   . Added IntlNumberRangeFormatter class to format an interval of two numbers with a given skeleton, locale, IntlNumberRangeFormatter::COLLAPSE_AUTO, IntlNumberRangeFormatter::COLLAPSE_NONE, IntlNumberRangeFormatter::COLLAPSE_UNIT, IntlNumberRangeFormatter::COLLAPSE_ALL collapse and
     IntlNumberRangeFormatter::IDENTITY_FALLBACK_SINGLE_VALUE, IntlNumberRangeFormatter::IDENTITY_FALLBACK_APPROXIMATELY_OR_SINGLE_VALUE, IntlNumberRangeFormatter::IDENTITY_FALLBACK_APPROXIMATELY and