Nicegui-pack: add --onedir support

[djongepier]

Hi all,

Since performance and likelihood of a false positive is way lower when using the '--onedir' option, could you add that to the nicegui-pack script?

Thanks for this awesome library/framework!

[falkoschindler]

Thanks for the suggestion, @djongepier!
I'm not too familiar with PyInstaller and its arguments. But if it helps to improve performance and reliability, we should definitely add the option. Should it even be a default?
Would you like to create a pull request? It should be straight forward in analogy to the other arguments. And this way you would become an official contributor of NiceGUI. 🙂