Don't add very long default values

[AlexWaygood]

Fixes #67

I've gone for the repr option for now, as I agree with @hauntsaninja that there doesn't seem to be a particularly obvious use case for incredibly long defaults. But, I went for a limit of 500 chars, as 100 chars felt overly conservative when I looked at it again.

[github-actions[bot]]

This change has no effect on how stubdefaulter would alter typeshed's stdlib. 🤖🎉