fix updating "code" prop for Editor component

[andgra]

The edited condition was never reachable because the default "state.prevCodeProp" is undefined and will never be updated because of this

[stevenocchipinti]

Pretty sure this fixes #279 and #281, I made this same change locally and it allowed me to use the code prop to control the state as I expected.

The documentation does state that the code prop doesn't include the users changes but I think it probably should as I couldn't find another way to dynamically change the state. Otherwise I'm guessing this probably should have followed the naming convention of defaultCode for uncontrolled props and code for controlled props (like defaultValue and value for <input>).

+1 for getting this merged in! If it does get merged, the docs might have to be updated too.

[jpdriver]

pushed a small update here that achieves the same thing since Editor has changed significantly since this PR was opened 🙇🏻‍♂️