[Suggest new feature] Register as a Preview Handler for Explorer

Suggested new feature: During installation, optionally set Notepad3 as the Preview Handler for Explorer, enabling preview of common text formats like .txt and .md in Explorer's preview pane.

For reference: https://github.com/MicrosoftDocs/win32/blob/docs/desktop-src/shell/how-to-register-a-preview-handler.md

Thanks :)