feat(model): add Model.applyVirtuals() to apply virtuals to a POJO

[vkarpov15]

Fix #14818

Summary

Logic largely copied from https://github.com/mongoosejs/mongoose-lean-virtuals/blob/master/index.js to apply a Model's virtuals to a POJO.

Examples

[hasezoey]

mostly looks good to me, some minor style suggestions, aside from that:

• typescript types for the new applyVirtuals are not added yet
• there is a distinct lack of documentation in the applyVirtuals file

[vkarpov15]

@hasezoey thanks for the feedback. I added docs and addressed issues 👍

[hasezoey]

Looks good to me, typescript types for the new applyVirtuals on Model are still missing though

[hasezoey]

Looks good to me