AK: Add is_owned() method to MaybeOwned

Author: https://github.com/MacDue
Commit: https://github.com/SerenityOS/serenity/commit/da00a5cdb5
Pull-request: https://github.com/SerenityOS/serenity/pull/21977
Reviewed-by: https://github.com/timschumi

AK/MaybeOwned.h

@@ -54,6 +54,8 @@ public:
     T& operator*() { return *ptr(); }
     T const& operator*() const { return *ptr(); }
 
+    bool is_owned() const { return m_handle.template has<NonnullOwnPtr<T>>(); }
+
 private:
     Variant<NonnullOwnPtr<T>, T*> m_handle;
 };