LibJS: Avoid pointless transitions and metadata lookups in storage_set()

- Replace the misleading abuse of the m_transitions_enabled flag for the
  fast path without lookup with a new m_initialized boolean that's set
  either by Heap::allocate() after calling the Object's initialize(), or
  by the GlobalObject in its special initialize_global_object(). This
  makes it work regardless of the shape's uniqueness.
- When we're adding a new property past the initialization phase,
  there's no need to do a second metadata lookup to retrieve the storage
  value offset - it's known to always be the shape's property count
  minus one. Also, instead of doing manual storage resizing and
  assignment via indexing, just use Vector::append().
- When we didn't add a new property but are overwriting an existing one,
  the property count and therefore storage value offset doesn't change,
  so we don't have to retrieve it either.

As a result, Object::set_shape() is now solely responsible for updating
the m_shape pointer and is not resizing storage anymore, so I moved it
into the header.

Userland/Libraries/LibJS/Runtime/Object.h

@@ -7,6 +7,7 @@
 
 #pragma once
 
+#include <AK/Badge.h>
 #include <AK/HashMap.h>
 #include <AK/String.h>
 #include <LibJS/Forward.h>
@@ -160,6 +161,9 @@ public:
     void enable_transitions() { m_transitions_enabled = true; }
     void disable_transitions() { m_transitions_enabled = false; }
 
+    void set_initialized(Badge<Heap>) { m_initialized = true; }
+    void set_initialized(Badge<GlobalObject>) { m_initialized = true; }
+
     template<typename T>
     bool fast_is() const = delete;
 
@@ -176,12 +180,13 @@ protected:
     bool m_has_parameter_map { false };
 
 private:
-    void set_shape(Shape&);
+    void set_shape(Shape& shape) { m_shape = &shape; }
 
     Object* prototype() { return shape().prototype(); }
     Object const* prototype() const { return shape().prototype(); }
 
     bool m_transitions_enabled { true };
+    bool m_initialized { false };
     Shape* m_shape { nullptr };
     Vector<Value> m_storage;
     IndexedProperties m_indexed_properties;