Document.QueryElementAll should return elements

Currently, this is a list of `Node`s requiring a type cast to get an actual element.

Maybe the node list implementation should be generic? Or maybe just a list of elements, and make it generic if we need it as a node list later on.

Start by locating Document.QueryElementAll and the node list implementation, then inspect any existing tests for query results. Resolve whether the API should return a list of elements or use a generic node list; done when the chosen behavior avoids the current type cast and is covered by tests.

Written by the indexing model from the issue text.