Add setting and command to show/hide cells in outline

[vezwork]

Fixes #647

Kapture.2026-05-08.at.12.51.44.mp4

• Adds a symbols.showCodeCellsInOutline setting to the Quarto configuration,
• Adds a quarto.toggleCodeCellsInOutline command to toggle that setting.

Note: I attempted to add tests but for some reason in the tests the outline / document symbols refuse to change. I tried many things including waiting longer during the test after calling quarto.toggleCodeCellsInOutline, programmatically getting document symbols with vscode.executeDocumentSymbolProvider, looking with my own eyes at the outline during the test. I'm not sure why it doesn't work in the test environment!

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