[SuperEditor][Adjustment] - Change TextNode and UpstreamDownstreamNode to implement EditableDocumentNode

[matthew-carroll]

[SuperEditor][Adjustment] - Change TextNode and UpstreamDownstreamNode to implement EditableDocumentNode

In a recent PR we introduced EditableDocumentNode, which introduces generic APIs for IME serialization and content editing, which should be able to apply to any type of node. For this ticket, migrate TextNode and UpstreamDownstreamNode to implement EditableDocumentNode and remove the node-specific editing behavior.