types: make toObject() and toJSON() not generic by default to avoid type widening

[vkarpov15]

Fix #12883

Summary

Making document methods generic by default makes TypeScript do some strange type widening where it automatically infers the generic parameter if the variable being assigned has an explicit type. This PR makes toJSON() and toObject() not generic by default.

In the interest of caution, I'm targeting merging this in v8.6 because it may cause some issues for users that rely on the broken behavior.

Examples

[hasezoey]

Looks good to me, would have to change something in typegoose as we seemingly relied on this behavior.

simplified example of what the use-case was:

interface User {
  name: string;
}

const pojo: User & { _id: mongoose.Types.ObjectId; id: string } = doc.toObject({ virtuals: true, getters: true });
// Error:
// Property 'id' is missing in type 'User & { _id: ObjectId; }' but required in type '{ _id: ObjectId; id: string; }'.

(TL;DR: we needed types for the default id virtual / getter, but it does not currently exist in mongoose yet, iirc)
link to actual typegoose source