[SuperEditor][SuperReader] - Minimize uses of node indices

[matthew-carroll]

When we change a Document from a list of nodes to a tree of nodes (#2433) the concept of a "node index" will change in a significant way. There will no longer be a global node index. Instead, a node will have an index within some parent node. The root node will be called the "document".

To prepare us for this change, convert every unnecessary use of node indices to some other node access mechanism, such that we can use the same access mechanism with a tree. For example, I expect that we'll retain getNodeById() at a document level, which means those uses won't be impacted by the change to a tree.