feat: implement graceful shutdown support with cancellation tokens

[leslierichardson95]

This pull request improves cancellation support and error handling across several command implementations in the docfx tool. The main changes ensure that long-running operations can be cancelled gracefully using a CancellationToken, and that the application returns appropriate exit codes and logs warnings when operations are cancelled.

Enhanced cancellation support and error handling:

• Updated command execution methods (BuildCommand, DefaultCommand, MergeCommand, PdfCommand, and DownloadCommand) to accept and properly handle CancellationToken, ensuring operations can be cancelled cleanly and return the correct exit code (130) when cancelled. [1] [2] [3] [4] [5]
• Modified CommandHelper.Run and CommandHelper.RunAsync to catch OperationCanceledException, log a warning, and return exit code 130 if cancelled, instead of -1 or 0.

Refactoring and consistency improvements:

• Refactored DownloadCommand to be asynchronous, use proper logging, and support cancellation, improving consistency with other commands.
• Added checks for cancellation before performing subsequent operations (e.g., serving, PDF creation) to avoid unnecessary work after cancellation is requested. [1] [2]

These changes make the command-line experience more robust, especially when running long builds or downloads that may need to be interrupted.