Add sortShapeProp option to sort-prop-types rule

[jomasti]

This allows the rule to enforce the sorting rules in a shape prop object.

I think it makes more sense to happen by default, but I expect it would be better to wait until the next major version for that. This PR should allow the rule to satisfy the request in #1476.

[ljharb]

LGTM overall.

In no way do I think that it should ever become the default; sorting prop names doesn't necessarily mean shape props should be sorted.

Separately, please add test cases that ensures that shape properties are not sorted such that they cross a spread boundary. Example that should not have an error:

Component.propTypes = {
  a: PropTypes.shape({
    d: PropTypes.any,
    ...someObject,
    a: PropTypes.any
  })
};

[jseminck]

What's the reason for this comment?

[jomasti]

I think I saw the comment as an intended marking of where the invalid test cases began and that it was in the wrong place. Looking back over it, the comment was actually in the right spot. I'll rebase to put this back.

[ljharb]

Please rebase this one more time; tests should pass now :-)