Export a Separate Type for PipelineStage Types Allowed in Facet Aggregation Sub-Pipelines

[mhoc]

Summary

This is a typescript type-only change.

Currently, the type definition for each sub-pipeline within a { $facet: ... } aggregation pipeline stage is baked in to the overall definition of the Facet stage.

We've got a situation where we'd like to pass these sub-pipeline arrays around, ideally in a type-safe way. Currently, that requires a bit of boilerplate to keep typescript happy; with this change, we should just be able to say, for example:

import * as mongoose from "mongoose";

export function Query(resultsFacet: mongoose.FacetPipelineStage[]) {
  return db.myCollection.aggregate([{
    $facet: { results: resultsFacet },
  }]);
}

Happy to change the name of this type if FacetPipelineStage doesn't make sense.

Thank you for the great project 😀