Improve/yjs document types

[alecgeatches]

WIP. Improve consistency of types stored in the Yjs synced document:

1. Change block types to Y.Map. Previously this was true for root blocks, but not for inner blocks.
2. Change innerBlocks use the Y.Array type to match root blocks. They were previously a Block[], which was different that the root blocks type (Y.Array< YBlock >).
3. Change rich-text types to use Y.Text. We'll probably need more logic to ensure relative position changes bind correctly to text updates, but this is a good start.

All of the consistency updates make parsing blocks more straightforward, and simplify types. This is an important step to implementing relative cursors, and typing within the same rich text block.

We've also improved the mergeCrdtBlocks() function to recurse and merge innerBlocks with the same strategy as root blocks.

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