AK+LibURL: Move CopyOnWrite<T> from LibURL to AK

AK/CopyOnWrite.h

@@ -0,0 +1,41 @@
+/*
+ * Copyright (c) 2024, Andreas Kling <andreas@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/NonnullRefPtr.h>
+
+namespace AK {
+
+template<typename T>
+class CopyOnWrite {
+public:
+    CopyOnWrite()
+        : m_value(adopt_ref(*new T))
+    {
+    }
+    T& mutable_value()
+    {
+        if (m_value->ref_count() > 1)
+            m_value = m_value->clone();
+        return *m_value;
+    }
+    T const& value() const { return *m_value; }
+
+    operator T const&() const { return value(); }
+    operator T&() { return mutable_value(); }
+
+    T const* operator->() const { return &value(); }
+    T* operator->() { return &mutable_value(); }
+
+    T const* ptr() const { return m_value.ptr(); }
+    T* ptr() { return m_value.ptr(); }
+
+private:
+    NonnullRefPtr<T> m_value;
+};
+
+}