Add toolchain for Node.js 20

[benfrancis]

For the WebThings Gateway 2.0 release we need to create a toolchain for Node.js 20.

I suspect we may need docker images with a base of Bookworm for this, rather than Buster.

As discussed in previous issues, we need to figure out how to do this without breaking add-ons for existing gateways.

(We'll need to do the same for a newer Python version too).