Updated header to include <auto-generated> key to exclude the file header to avoid compilation warnings.

[Taron-art]

Thanks for a nice library. Current version creates build warnings related to nullables. This change fixes this by marking the code as generated (similar to other generators).

Changes:

• Header was updated to include key;
• WithUsings and WithLeadingTrivia were reversed since otherwise the comment is at the end of the file.