Drag handle improvements

If you drag an element into another element such as a list it's not a top level element anymore and therefore cannot be dragged anymore.

At first sight there seem to be two options to solve this:

1. Do not allow dragging content into other elements - only allow sorting.
2. Allow dragging subelements such as nested lists.

I don't know if 1. is feasible and it would of course reduce the usefulness of the feature - on the other hand it would also make it easier / more clear to use.

I don't know if 2. is feasible. It would be nice to be able to sort lists with this as well for example. But it's hard to destinguish if i want to drag the first list item or the entire list. I imagine there's more such questions this would raise due to the increased complexity.

Originally posted by @max-nextcloud in https://github.com/nextcloud/text/issues/7537#issuecomment-3195137387

No file, test, or entry point is named. Start by locating the editor's drag-handle behavior and reproduce dragging a top-level element into a list, then assess the two proposed behaviors and their ambiguity for nested lists. Done means one behavior is selected, implemented, and covered by tests for dragging nested content.