fix: Discriminator generic type not being passed to schema

[GCastilho]

-- Fix schema type passed to discriminator method not extending Document

Summary

A model created from a discriminator does not have types for the "document" (save, find, etc) as the original model does

Examples

# Old
const ClickedLinkEvent = Event.discriminator('ClickedLink', new mongoose.Schema({ url: String }, options));
ClickedLinkEvent.save() // save has type any

# New
ClickedLinkEvent.save() // save has the type from Document['save']