add domAtPos to support leaf nodes

[eshvedai]

Currently view.domAtPos is working as expected for block nodes with content (e.g. a paragraph or a table).

However, for leaf nodes (atom node without any content, either nested inside other nodes or at the top level) it returns the DOM reference of its parent node with an offset.

The desired behaviour would be to always return a DOM reference that corresponds to the given document position.