Support for comparator on isDescendant

So I have come across a situation where I need to check if a folder is a descendant of another folder, but the object has changed shape but the ID is the same. Currently, isDescendant is comparing the references of the two objects. I would like to add a feature that would support the ability for the user to define their own comparing method (is comparator the right terminology here?).

```js
// example implementation (untested)
export function isDescendant(older, younger, comparator) {
return (
!!older.children &&
typeof older.children !== 'function' &&
older.children.some((child) => {
if (comparator && typeof comparator === 'function') {
return comparator(older, younger) || isDescendant(child, younger, comparator)
} else {
return child === younger || isDescendant(child, younger)
}
})
)
}

// example usage
isDescendant(node1, node2, (parent, child) => {
return parent.id === child.parent_id
})
```

If anyone is interested I can spend some time thinking more into this and submit a PR.

Start by locating the isDescendant entry point and inspect how its current reference comparison is covered. Clarify the comparator API against the example, then add coverage for matching changed-shape nodes while preserving reference-based behavior when no comparator is supplied. Done means the new option works for the parent/child relationship described and existing callers remain valid.

Written by the indexing model from the issue text.