[New addon] Stepping in Scratch editor about scratchaddons

[WorldLanguages]

Okay I think the API is working now, wanna try to add your addon?

1. Create an editor-stepping folder inside the addons folder.
2. Add "editor-stepping" to addons.json
3. Add an addon.json file to your addon's folder, you can use editor-devtools's manifest as a base (IMPORTANT: make sure to set enabled_by_default to true, so that you don't have to manually enable it inside the background script's console)
4. Have a userscript.js that goes like this:

export default async function({ addon, global, console }) {
  const vm = await addon.tab.getScratchVM();
  // Do whatever with vm
}

5. Reload the extension on chrome://extensions and hopefully it works :)
   Tag me if you need help.

[WorldLanguages]

This "find vm" approach should probably be made by the addon loader so that many addons can access the VM object (and not just this one). Related to #17

[BoomerScratch]

@WorldLanguages addon.ScratchVM?

[WorldLanguages]

@BoomerScratch Yeah, it will probably be addon.tab.getScratchVM (returns a promise) and will resolve when it finds the VM.