[Php86] Add ConstructorReadonlyAssignToDefaultRector for readonly property defaults

[TomasVotruba]

PHP 8.6 allows default values on readonly properties.

This rule moves a constant constructor assignment of a readonly property to a property default value, and drops the now-redundant assignment.

 final class SomeClass
 {
-    public readonly int $number;
+    public readonly int $number = 100;

     public function __construct()
     {
-        $this->number = 100;
     }
 }

Narrow by design - only fires when:

• the property is readonly, non-static, single, without an existing default;
• the constructor has a top-level assignment of a constant expression (scalar, const/class-const fetch, unary +/- of those);
• variable/param/new-object assignments and conditional (nested) assignments are skipped.