feat: print full stack trace on error

[tmillr]

• What you're trying to do
  Debug my code.

• Why you can't use AVA for this
  No stack trace on error? Only error message is printed.

• And maybe how you think AVA could handle this
  Print full stack trace on error so that I can find out where they are occurring, including when they occur within async fn's. Or, at the very least make it opt-in.

Things can get really painful when errors occur and there's only an error message but no stack trace. I'm not sure if this is something specific to async functions and/or specific to AVA, but it can get pretty annoying when rapidly prototyping with vanilla JS. Is this the expected behavior? Or maybe I've messed something up?