✨ feat(__main__.py): add optional static_files_dir parameter to setup_app function

The variable name static_files_dir was changed to improve semantics. It is now more clear that it is a directory path. An optional static_files_dir parameter was added to the setup_app function to allow for a directory path to be passed in. This allows for more flexibility in serving static files.
🐛 fix(__main__.py): fix static_files_dir variable name to improve semantics