registerCommand (commandName, fn) {
    this.#hooks.commands[ commandName ] = fn
  }
Register a command that will become available as
`quasar run <ext-id> <cmd> [args]` and `quasar <ext-id> <cmd> [args]`

@param {string} commandName
@param {function} fn
  ({ args: [ string, ... ], params: {object} }) => ?Promise