Improve the ViewModel editor

[phillipskevin]

Right now the ViewModel editor is really barebones. Some things that would make it a lot more useful

• [X] make editing non-strings possible (numbers, array, etc)
• [ ] integrate new JSON editor
• [ ] add a "break on change" button
• [ ] figure out what to do with Promises
• [ ] don't allow people to edit non-writable properties

[phillipskevin]

All of these things are handled in separate issues, and the overall change is part of https://github.com/canjs/canjs/issues/4321, so going to close this.