feat(playground): export Y.Doc from debug menu (#14893)

## Summary
- add an Export Y.Doc debug menu item
- encode the active store spaceDoc with Y.encodeStateAsUpdate
- download the update as a binary ydoc-update file

## Test
- yarn workspace @blocksuite/playground build

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## Summary by CodeRabbit

* **New Features**
* Added Y.Doc export functionality to the debug menu, enabling users to
download the current space document state as a binary update file
through the "Test Operations" → "Export" menu.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->