Setup Repo

[cptpiepmatz]

This PR sets up the repo with everything we talked about for example plugins in our meeting.

It adds the following plugins:

• nu_plugin_node_example
• nu_plugin_nu_example
• nu_plugin_python_example
• nu_plugin_custom_values
• nu_plugin_example

I’ve barely touched them, so they should behave just like before.

To make version handling easier, I moved the versions into their own files. The .python-version and .node-version files should make sure script-based plugins run with the expected runtime versions.

[fdncred]

Thanks for working on this. Feel free to land it when you're ready.