feat: Fix selection and remove text cursor position

[matthewlipski]

getSelection now fully replaces getTextCursorPosition and has been improved in the following ways:

• No longer returns all descendants.
• Also includes prevBlock, nextBlock, and parentBlock.
• Should be more performant as we no longer traverse all nodes in the doc.

setSelection now fully replaces setTextCursorPosition and has been improved in the following ways:

• You can now set the selection to span multiple blocks.
• When setting the selection to a single block, you can now set it to span the content of that block.

Additionally, moving blocks up and now works with selections spanning multiple blocks.

Closes #1239

TODO: Docs

[vercel[bot]]

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