Yep I was suspecting it’s not officially there yet. I am also working on something beta, so I am using this just to demostrate the functionality of the editor.
With the latest commits, you can go about it in two ways. If you just want to make a minor change from the default menu, you can add your own commands using defineCommand, and giving them a menuGroup property. To remove default menu items, you can use the commands option, something like this:
To completely replace the content of the menu, you can either switch it over to a new set of menu groups, using the groups: ["mygroup"] config option, or directly give a set of command names using items: ["command1", "command2"].