Allow ESM in danger-js code

[TomStrepsil]

When babel-preset-asos was introduced (to allow the repository to be open-sourced, in version 0.1.0 of the root repository), an issue with danger-js was discovered.

The babel-preset-asos code could not itself be processed by babel, otherwise a circular reference / call stack max-out would occur. This meant the preset had to be ignored, via the babel config.

Unfortunately, danger-js did not handle ignored files well when it transpiled its source or any supplied plugins, meaning a concession was made to convert all of this to commonJs format code, that would run natively in Node.

Having fixed the issue with danger-js, this repo can be put back to its original form.