fix: "this" of Schema's instance methods

[ruxxzebre]

Summary

Fixes bug described in this issue.

Examples
Before

After

[pshaddel]

@ruxxzebre Looks nice! Could you implement some tests that check the type of this in methods, both when you are not passing InstanceMethods and when you are passing your desired instance?

[pshaddel]

@ruxxzebre Also please look at this line:
https://github.com/Automattic/mongoose/blob/master/types/index.d.ts#L300

I guess we do not have typing in this situation because of this line:

schema.methods.fullName = function fullName() {
    expectType<string>(this.firstName);
    return ''
  }

And maybe re-use your implementation so the type of this methods stays consistent.

[ruxxzebre]

I seem to have found similar behaviour with statics, as well as query helpers.
For query helpers there's example in docs on how it can be mitigated.

ProjectSchema.query.byName = function byName(
  this: QueryWithHelpers<any, HydratedDocument<Project>, ProjectQueryHelpers>,
  name: string
) {
  return this.find({ name: name });
};

With AddThisParameter we can help with typings of those also. What you think?

[pshaddel]

@ruxxzebre PR Looks nice. Yes, if you can fix those two, why not trying it! 😊
BTW I am not a maintainer so for the merge and final review you should wait for a maintainer to look at this. But good job fixing the issue!

[vkarpov15]

This is excellent, thanks for this PR. We've been struggling to figure out how to implement something like the AddThisParameter<> helper 👍

[vkarpov15]

Feel free to put in a separate PR for the query helper changes you mentioned in #14028 (comment)