Export types to be able to extend from the Mongoose class

### Prerequisites

- [X] I have written a descriptive issue title
- [X] I have searched existing issues to ensure the bug has not already been reported

### Mongoose version

6.6.1

### Node.js version

18.x

### MongoDB server version

5.x

### Description

Currently the type definitions only export ``mongoose`` as module but don't allow to import the actual ``Mongoose`` class so that we can extend in TS from.

### Steps to Reproduce

``` typescript
import {Mongoose} from 'mongoose'

export class Database extends Mongoose {
....

public async connect(): Promise {
...

await supper.connect(...);
return true;
}
}
```

### Expected Behavior

I'm able to extend the base Mongoose class within TypeScript.

Start by locating the TypeScript type definitions and the exported module entry point for Mongoose. Check how the Mongoose class is currently declared and exported, then verify that `import { Mongoose } from 'mongoose'` can be extended by a TypeScript class and that the resulting type definitions compile.

Written by the indexing model from the issue text.