LibJS: Add Object::set_prototype()

This is just factoring out step "9. Set O.[[Prototype]] to V." of
10.1.2 [[SetPrototypeOf]] into its own method so that we don't have to
use internal_set_prototype_of() for setting an object prototype in all
cases.

Userland/Libraries/LibJS/Runtime/Object.h

@@ -171,6 +171,8 @@ protected:
     explicit Object(GlobalObjectTag);
     Object(ConstructWithoutPrototypeTag, GlobalObject&);
 
+    void set_prototype(Object*);
+
     // [[Extensible]]
     bool m_is_extensible { true };