Expose Aggregate.count(countName) method

The Aggregate class should expose a `count(countName)` method that adds a `$count` aggregator in the pipeline.

Documented here: http://mongoosejs.com/docs/api.html#aggregate_Aggregate-count
Code @5.0.15 referencing this method: https://github.com/Automattic/mongoose/blob/5.0.15/lib/aggregate.js#L379

types/mongoose/index.d.ts

@@ -2274,6 +2274,12 @@ declare module "mongoose" {
     /** Adds a collation. */
     collation(options: CollationOptions): this;
 
+    /**
+     * Appends a new $count operator to this aggregate pipeline.
+     * @param countName name of the count field
+     */
+    count(countName: string): this;
+
     /**
      * Sets the cursor option option for the aggregation query (ignored for < 2.6.0).
      * Note the different syntax below: .exec() returns a cursor object, and no callback