feat: cleanup generated folder

[jpudysz]

When I was experimenting with some nitrogen options, I noticed that the generated folder retains old directories and doesn't remove them when there is no spec.

This small PR ensures that the generated folder will be removed recursively whenever the user runs script. Additionally, if the user decides to remove specs and runs the script again, the folder will be empty.

Before:

Example if I try to generate code from swift / kotlin to C++

After:

Also there is a small indicator that folder has been removed:

[mrousavy]

Thanks for this PR, this is definitely fixing a bug, but I explicitly scanned the directory first, wrote the files, and only AFTERWARDS I removed all files that weren't part of the generated specs.

That way we can avoid flickers in IDEs or git tools or watchman, overwriting files is better than deleting and writing again.

But maybe I am overengineering this...
I think we cannot merge this as is, we need to remove directories the same way we remove unused files at the end of index.ts

on my phone rn so cant post line numbers