feat: support extending type for mongoose.models

[MunifTanjim]

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Summary

Currently mongoose.models can not be explictly typed. So, mongoose.models.Cat will always have the type Model<any>.

This change enables users to explicitly type mongoose.models.

Examples

Create a file mongoose.d.ts in your codebase:

declare module 'mongoose' {
  interface Models {
    Cat: CatModel; // CatModel is something like `interface CatModel extends Model<CatDocument>`
  }
}

Now the type for mongoose.models.Cat will be CatModel, type for mongoose.models.Fish will be Model<any>

[vkarpov15]

Thanks 👍