feat(ts): Export MongooseError class

[ramos-ph]

Summary

Exports the class MongooseError so it can be tested using err instanceof MongooseError

Examples

import mongoose from 'mongoose';

const Model = mongoose.model('model', new mongoose.Schema({ answer: Number }));

async function main() {
  const doc = new Model({ answer: 'not a number' });
  const err = doc.validateSync();

  console.log(err instanceof mongoose.Error); // true
  console.log(err instanceof mongoose.MongooseError); // true
  console.log(err instanceof mongoose.Error.ValidationError); // true
}

main();

Closes #13387

[vkarpov15]

The code for this change looks fine, but I don't think this is the right approach for fixing #13387. Like OP said, the ideal approach would be just to check if err instanceof MongooseError. So I think the better approach would be to make sure MongooseError is exported for use in TypeScript.

[vkarpov15]

LGTM 👍 I'll merge this into 7.2 branch.