Strongly typed next function.

[WouterSpaak]

Hello @jvdieten!

Thank you for this little library, we're using it for end-to-end tests in various places where we generate XML files for geospatial analysis.

In this PR, I changed the type signature of the next argument to be next: (result: IDiffResultModel[]) => void, to ensure that any function passed as the final argument to diff and diffAsXml will get the correct type information.

This shouldn't be a breaking change, and if it is for downstream users, they've probably got a bug anyway...

I was also going to alter the type signature of the functions to return something generic, like:

export function diffAsXml<R>(
  lhs: string,
  rhs: string,
  schema: ISchema | undefined,
  options: IOptionsModel | undefined,
  next: (result: IDiffResultModel[]) => R
): R {
  return next(compareObjects(...));
}

Then you could use it like so:

const result = diffAsXml(lhs, rhs, undefined, undefined, (r) => {
  return 'I am the result of the callback!';
});

console.log(result); // 'I am the result of the callback!'

However I feel that may be too much of an impact to the API. If you feel this would be valuable, I'd be more than happy to create a seperate pull request for that.

Kind regards,
Wouter