Allow *any* bookmark tree structure (separate from DOM structure)

Is it possible to define my own bookmark tree?

It would be nice to have an API like this:

```
const pdf_doc = () => {
const bookmarks = {
"title 1": {
"title 2": {}
}
};

return

{
bookmarks["title 1"]["title 2"] = el
}}>

}
```

Explanation:
- Map JSON's tree structure to the bookmark structure.
- Give the object ref to the ``, which after rendering has the complete bookmarks object and rendered page locations and can add the final bookmark listing to the PDF.
- Allow any PDF elements to be placed manually by the user to one or many locations in the tree.

The issue I have with the current bookmarks is that it takes the location in the DOM - it only allows linear book-chapter-like bookmarks. It also works unpredictably with auto page wrapping.

I need to create an index of topics - one element can be in many topics.

Thanks for the great lib and have a good day 🛸