Please add a sample for using Editor Menus and Submenus

[lokeshgoel177]

We have a settings Menu + Submenu in case of Notebook files. I was looking for some similar submenu page for some particular extension types but this doesn't seem to be straight-forward like other API's. There is very minimal information about this in Docs as well. Having a dedicated sample will help ease the usage of this feature.

[Dulatr]

I would echo this, the example given in #873 does not work for me. I can't get pretty much any version between docs and the github issues to show a submenu like imaged

[Dulatr]

In true fashion, it was just a skill issue on my part... I was attempting to use the explorer/context menu but was using the editor/context menu to get them to show up (embarassing, but I blame lack of sleep). The sample in #873 ultimately worked in that aspect. The docs though aren't as clear as described in that github issue. What I saw through documentation was to add the submenu declaration but didn't describe how to add the submenu to your specific context menu.