Unable to declare type of "this" in function

Right now the style rules make it impossible to declare the type of "this" in a function.
When you do it like this:

```typescript
function (this: UserDocument) {
}
```
it tells you to use an arrow function (Unexpected function expression.(prefer-arrow-callback)) that do not support to set "this".

I need this for mongoose virtuals where you can set virtual fields to the value returned by a callback function. You can access all document fields from "this" inside the function. In my use case, there is a virtual field named "makeJwt" which needs access to the jwtSecret stored in the UserDocument.

Is it intended to disallow this in functions in general?