Support mjs plugins

In the documentation for plugins it shows that you can write plugins by exporting a Plugin interface.

Inconsistent documentation for plugin mjs

First, the documentation shows Plugin interface with ESM, but immediately below the example shows module.exports = definePlugin which is CommonJS and seems inconsistent.

Error with plugin mjs

Also, I have tried implementing this using ESM, please see a minimal test case at:

https://github.com/fulldecent/github-pages-template/tree/c2c394068d17bd6a939ba173a035dcb2183ccf33

and this produces errors about FileSystemConfigLoader.loadFromFile failing to load the plugin.