Remove need for app.json to specify pr-predestroy script

[douglascayers]

Just as Heroku CI will automatically look for bin/test-compile and bin/test scripts, if it's possible for Heroku during review app destroy step to look for something like bin/pr-predestroy script then that would allow the buildpack to provide a default and developers not to have this knowledge in their app.json unless they wanted to customize the behavior.

Currently, developers are adding the below snippet to their app.json, which relies on underlying knowledge of what artifacts this buildpack generates.

[wadewegner]

I don't think this is possible, which is why I have it setup to use the app.json. I totally agree that it requires an absurd amount of knowledge and that it's easy to miss.

@ike-delorenzo and @devarispbrown, do you have an item on the roadmap to provide a better way we can hook this in, ideally through the buildpack itself so it doesn't require anything by the developer?