Add clean script

[Hornwitser]

It would be useful to have a clean script which deletes the {packages,plugins}/*/dist folders and can be used before publishing to make sure no leftover files from previous builds are accidentally included into the published packages.

This script should ideally work on both Windows and Linux, and be documented as part of the publishing procedure in docs/contributing.md.

[Danielv123]

How about making a CI task for publishing instead? That reduces the chance of human error significantly.