Better type inference for egg-mongoose

[Celend]

Hi!, We're trying to use egg.js in production with mongoose, but we notice a problem about the typing for Mongoose with egg-ts-mongoose plugin.

Mongoose itself has the typing solution(separate interface declaration or use ts-mongoose), and the egg-ts-helper also generated a correct type file of model files(interface IModel), Awesome!
But when using the model in service by this.ctx.model, the model type is referred to the egg.Context.model in egg-ts-mongoose/index.d.ts(https://github.com/eggjs/egg-mongoose/blob/master/index.d.ts#L28), which type is [key: string]: mongoose.Model<any>.

So in the service, the model type is Model<any>, it's very stupid! and the solution also very easy, just override the type of context.model to IModel, by edit typing/index.d.ts as below:

import 'egg';

declare module 'egg' {
    // add below lines
    interface Context {
        model: IModel
    }
}

(Edit the typing/model/index.d.ts should has same result)

I think this job can not be done by egg-mongoose, because the interface IModel is a generated file.

I'm using WebStorm 2020.2, but I think the VSCode should has the same problem, because the context.model type refer to egg-mongoose seems a correct behavior or maybe I wrong :D

Thanks!