## Summary
Previously, `ruff.applyFormat`, seen in VS Code as the command `Ruff:
Format Document`, would only format the currently active notebook cell
inside a notebook document. This PR makes `ruff.applyFormat` format the
entire notebook document at once, operating on each code cell in order.
## Test Plan
1. Open a notebook document that has multiple unformatted code cells.
2. Run `Ruff: Format Document` through the Command Palette
(`Ctrl/Cmd+Shift+P` by default)
3. Observe that all code cells in the notebook have been formatted.
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