improve sandbox command error handling and debugging

[yannbf]

What I did

The yarn sandbox command was failing silently when the sandbox folder didn't exist. This PR does:

• Ensure sandbox folder always exists before doing anything
• Replace shelljs implementation with execa
• Add "debug" flag so users can get full output, which is set to false by default to all steps but yarn storybook. The result is nice:

Additionally, if you pass a wrong value to the possible values of a flag, you get a much better response now:

How to test

1. have no sandbox folder
2. run yarn sandbox

• Is this testable with Jest or Chromatic screenshots?
• Does this need a new example in the kitchen sink apps?
• Does this need an update to the documentation?

If your answer is yes to any of these, please make sure to include it in your PR.

[tmeasday]

Love the debug flag. I think the execa call needs to actually error and also maybe show the output when it fails though.

[tmeasday]

Awesome!