fix: Discriminator generic type not being passed to schema

[GCastilho]

-- Fix instance created from model returned from discriminator having type 'any'
-- Fix model created from discriminator not inheriting the properties from the base model

Summary

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

Examples

type EventSchema = {
    time: Date
}
const eventSchema = new mongoose.Schema<EventSchema>({ time: Date }, options);
const Event = mongoose.model('Event', eventSchema);

type ClickedLinkEventSchema = {
    url: string
}
const clickedLinkEventSchema = new mongoose.Schema<ClickedLinkEventSchema>({ url: String }, options)
const ClickedLinkEvent = Event.discriminator('ClickedLink', clickedLinkEventSchema);
const clickedLinkEvent = new ClickedLinkEvent({
    time: new Date(),
    url: 'https://mongoosejs.com/',
});

# Old
clickedLinkEvent.time  // time has type any
clickedLinkEvent.save() // save has type any

# New
clickedLinkEvent.time  // time has type Date
clickedLinkEvent.save() // save has the type from Document['save']

This is a remake of #11897 with fixed branch name and commits

[vkarpov15]

Tests are failing because you also need to modify types/schematypes.d.ts, there's a couple places in that file where we use AcceptsDiscriminator:

mongoose/types/schematypes.d.ts

Line 272 in 4cf2920

class Array extends SchemaType implements AcceptsDiscriminator {

for example.

[GCastilho]

I've updated the classes in types/schematypes.d.ts that implemented the AcceptsDiscriminator to be generics

Their 'B' type is passed to SchemaType and Schema so I believe it is "the type of the class", so "an Array of numbers", using your example

It defaults to any so no change in behavior is expected

[vkarpov15]

Thanks 👍

[vkarpov15]

In hindsight, I think I'm going to have to revert this PR. This really slows down our TS performance benchmark.

Before:

Run node ./benchmarks/typescript
simple instantiations: 42294
simple memory used: 1[6](https://github.com/Automattic/mongoose/runs/6776724234?check_suite_focus=true#step:5:7)5158.9
simple check time: 2.[7](https://github.com/Automattic/mongoose/runs/6776724234?check_suite_focus=true#step:5:8)21[9](https://github.com/Automattic/mongoose/runs/6776724234?check_suite_focus=true#step:5:10)999999999995
simple total time: 4.175999999999999

After:

Run node ./benchmarks/typescript
simple instantiations: 2557[1](https://github.com/Automattic/mongoose/runs/6747560604?check_suite_focus=true#step:5:1)4
simple memory used: 19440[7](https://github.com/Automattic/mongoose/runs/6747560604?check_suite_focus=true#step:5:8).2
simple check time: 3.99[8](https://github.com/Automattic/mongoose/runs/6747560604?check_suite_focus=true#step:5:9)
simple total time: 5.448

[GCastilho]

Well, it does make sense that performance would slow down a little since the types were being defaulted to any and with this PR some generics were added for the types to be passed down

It is a new "feature", per se, it make sense for it to impact something