Prepare for JupyterLab 3.0 version update

[jahn96]

Purpose: Update code snippets extension to be compatible with JupyterLab 3.0 release.

Tasks:

• Require Node v12
• Use update script
• Fix encountered bugs

[dhendry]

This is one of my favourite extensions. Would love to see it continue to be maintained and updated for JupyterLab 3.0!

Thanks a lot for the great work!