AK: Add the SetOnce class

The SetOnce class is meant to be used as one-time set boolean flag,
which is useful for flags that change only once and then stay immutable
forever.

AK/SetOnce.h

@@ -0,0 +1,36 @@
+/*
+ * Copyright (c) 2024, Liav A. <liavalb@hotmail.co.il>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Noncopyable.h>
+#include <AK/Types.h>
+
+namespace AK {
+
+class SetOnce {
+    AK_MAKE_NONCOPYABLE(SetOnce);
+    AK_MAKE_NONMOVABLE(SetOnce);
+
+public:
+    SetOnce() = default;
+
+    void set()
+    {
+        m_value = true;
+    }
+
+    bool was_set() const { return m_value; }
+
+private:
+    bool m_value { false };
+};
+
+}
+
+#if USING_AK_GLOBALLY
+using AK::SetOnce;
+#endif