[mini-code-edit] Added back command Ctrl+Shift+1

The `commands` API is now stable according to https://codereview.chromium.org/253593003. Let's re-add back the commands key to the mini-code-edit manifest since it is already taking of it: https://github.com/GoogleChrome/chrome-app-samples/blob/master/mini-code-edit/background.js#L13-L13