fix: allow nested schemas on discriminated models to work

[DesignByOnyx]

Summary

This allows nested schemas on discriminated models to work (closes #42). While implementing the fix, I was able to rip out a lot of unnecessary code. I did not intend on such a refactor, but the resulting code feels much cleaner and doesn't need to worry about projection. I ran the tests against mongoose 7.7.0, 8.5.0, and 8.5.1.

Examples

Please see the test code and code referenced in the open issue #42.

[DesignByOnyx]

Apologies for bombarding you guys with a bunch of issues and PRs on this repo. All of the contributions I've made have come from a real project which is used for some really big companies. I've been upgrading our codebase from mongoose 6 to mongoose 8, and this package from 0.4.0 to 2.1.0. I had the bandwidth to make these changes and have written tests and tested it in a real-world app.

All of my PRs will close every open issue on this repo, and I am actively using all of these changes in our project.

I would love to get these changes merged so that we don't have to maintain our own fork (and so that you don't encounter these issues from other users). Please let me know what I can do to push these along. I'm able and willing to address any feedback.

[vkarpov15]

This code is very helpful, thanks, just has some merge conflicts that we will clean up in master after merging because it's easier to make changes locally vs GitHub.