Add nvm installation and utilisation to nodejs role

[gregharvey]

We've had a few instances of the default node version inexplicably changing. The Internet seems to agree that one should use the nvm utility to manage that:

• https://github.com/nvm-sh/nvm
• https://www.digitalocean.com/community/tutorials/how-to-install-node-js-on-debian-10#installing-node-using-the-node-version-manager

Should be easy enough to add this to servers with node on to make life easier.