fix(document): allow calling $assertPopulated() with values to better support manual population

[vkarpov15]

Fix #12233

Summary

#12233 highlighted a gap in $assertPopulated(): there's no way to manually populate using =.

// TypeScript complains about incorrect type
newParent.nestedChild = new NestedChildModel({ name: 'NestedChild' })

newParent.$assertPopulated<{ nestedChild: NestedChildModel }>('nestedChild')

// Throws at runtime because `nestedChild` isn't populated yet
newParent.$assertPopulated<{ nestedChild: NestedChildModel }>('nestedChild')

newParent.nestedChild = new NestedChildModel({ name: 'NestedChild' })

This PR combines the two steps into 1 for TypeScript's benefit:

// Does `$assertPopulated()` and `$set()` in one operation
const doc = newParent.$assertPopulated<{nestedChild: NestedChild}>('nestedChild', {
  nestedChild: new NestedChildModel({ name: 'NestedChild' })
})

What do you think of this @ajwootto ?

Examples

[AbdelrahmanHafez]

LGTM, thanks. 👍

[hasezoey]

LGTM