feat(types+document): add $assertPopulated() for working with manually populated paths in TypeScript

[vkarpov15]

Fix #11758

Summary

Right now, if you instantiate a document with manually populated paths, there's no way to indicate that a path is manually populated as opposed to just an ObjectId.

const parent = new ParentModel({
    nestedChild: new NestedChildModel({ name: 'test' }),
    name: 'Parent'
})

parent.nestedChild; // always ObjectId, even if manually populated

With this PR, you can at least do:

const parent = new ParentModel({
    nestedChild: new NestedChildModel({ name: 'test' }),
    name: 'Parent'
}).$assertPopulated<{ nestedChild: NestedChild }>('nestedChild')

doc.nestedChild.name; // string

which will make TypeScript work correctly, and also correctly throw a runtime error if 'nestedChild' isn't populated. What do you think @AbdelrahmanHafez @Uzlopak ?

Examples

[AbdelrahmanHafez]

LGTM, thanks! 👍

One minor note though, it might be a good idea to give this label of typescript as well, because I first assumed it was a JS fix, not a mostly TS one.