Fix text content in inline embedded nodes

[jmoggr]

Note: I leaned heavily on LLMs for this. I'm confident in my understanding of the problem, but I am not confident in understanding my proposed fix in the context of the whole project.

Replaying automerge spans produced incorrect prosemirror documents when inline embedded nodes contain text. Text content inside the node would move outside the inline embedded node as a sibling after a save/load cycle. In traverseSpans, the newBlock method treats all embed blocks as leaf nodes so subsequent text spans are then added to the parent container instead of the embed.

This is fixed by checking if the node is lineline and can contain text then the nodes is pushed so that subsequent text becomes children.

This is "stacked" on top of #56 and #55