Began porting my MultiPageViewModel from the Windows Forms library to the Avalonia library; updated copyright statement for the year 2026

Source/Commands/IAsyncCommand.cs

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+#region Apache License 2.0
+/*
+Nuclex Foundation libraries for .NET
+Copyright (C) 2002-2026 Markus Ewald / Nuclex Development Labs
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+#endregion // Apache License 2.0
+
+using System;
+using System.Threading.Tasks;
+using System.Windows.Input;
+
+namespace Nuclex.Avalonia.Commands {
+
+  /// <summary>Command that executes asynchronously</summary>
+  /// <typeparam name="TArgument">Type of an optional argument for the command</typeparam>
+  public interface IAsyncCommand<TArgument> : ICommand {
+
+    /// <summary>Whether the command is currently executing</summary>
+    bool IsRunning { get; }
+
+    /// <summary>Runs the command asynchronously</summary>
+    /// <param name="argument">Optional argument for the command</param>
+    /// <returns>A task that completes when the command has executed</returns>
+    Task ExecuteAsync(TArgument argument);
+
+    /// <summary>Triggers reevaluation of <see cref="ICommand.CanExecute(object)" /></summary>
+    void NotifyCanExecuteChanged();
+
+  }
+
+} // namespace Nuclex.Avalonia.Commands