fix(model): add results property to unordered insertMany() to make it easy to identify exactly which documents were inserted

[vkarpov15]

Fix #12791

Summary

#12791 pointed out that it is still very tricky to figure out exactly which documents were inserted if you're using insertMany() with ordered: false. That's because the document can either 1) fail Mongoose validation, 2) fail because of a MongoDB server error, 3) succeed. And the results for (1), (2), and (3) are all in separate properties, which means you need to do some stitching to figure out exactly what happened with a particular document.

With this PR, if there were any errors, there will be a results property on the error (or res.mongoose.results if you're using rawResult: true) that contains an array which has either 1) the document if the document was successfully inserted, 2) the MongoDB write error if the write failed, 3) the Mongoose validation error if the document failed validation. The index in the results array corresponds to the index of the original document in arr in the Model.insertMany(arr) call, so this should make it easy to determine if an individual document succeeded or not in insertMany(). Just check error.results[i] instanceof mongoose.Document.

Examples

[hasezoey]

Looks good to me, but documentation and types are missing

[vkarpov15]

@hasezoey take a look now, I added types and docs, and fixed some test issues