Add usePushEach flag to Mongoose Schema Options

In order for devs to be able to work around the issues described in
https://github.com/Automattic/mongoose/issues/5574 we need to be able
to supply the `usePushEach` flag to the Mongoose SchemaOptions. This
options functionality is described in
https://github.com/Automattic/mongoose/issues/4455 and is required for
anyone who modifies arrays inline who is using Mongoose 4 and MongoDB
3.6+

Testing:
  Copied changes to local project and verified compilation worked.

types/mongoose/index.d.ts

@@ -815,6 +815,8 @@ declare module "mongoose" {
     typeKey?: string;
     /** defaults to false */
     useNestedStrict?: boolean;
+    /** defaults to false */
+    usePushEach?: boolean;
     /** defaults to true */
     validateBeforeSave?: boolean;
     /** defaults to "__v" */