feat: allow defining virtuals on arrays, not just array elements

[vkarpov15]

Summary

Neat little feature that allows defining virtuals on arrays, so you can define a last virtual on an array that gets you the last element in the array:

schema.virtual('nums.last', { applyToArray: true }).get(function() {
  return this[this.length - 1];
});

// Later
doc.nums.last; // last element in the array

Fix #2326

Examples

[hasezoey]

just to make sure, these 2 ways (.path().virtual() & .virtual('path', applyToArray)) are equivalent?

[vkarpov15]

Yes they are equivalent.