fix: allow calling parent() on parent, allow accessing parent's virtual from child subdoc

[vkarpov15]

Fix #65
Fix #64

Summary

#64 points out that, because mongoose-lean-virtual applies virtuals to child subdocs before their parents, that makes it tricky to use parent virtuals in child subdocs. This change was intentional with #28, and we can't change the subdoc traversal order without a backwards breaking change. But, given that parent() is the only way to get a subdocument's parent, mongoose-lean-virtuals can apply virtuals in the parent() call to ensure the child subdoc gets the parent doc with virtuals applied.

This can create issues if a subdocument calls parent(), and then the parent tries to access one of the subdocument's virtuals. The subdocument virtual won't exist yet. But this case is hopefully unlikely.

I also bumped ESLint, fixed some lint warnings, added parent() to TypeScript type defs, and fixed #64 (also #66) because that's just a one-line fix.

Examples