Support notebook-scoped VS Code actions

[charliermarsh]

Summary

VS Code added support for notebook.*-scoped code actions (https://code.visualstudio.com/api/references/vscode-api#CodeActionKind), which are intended to run over the entire document, unlike the actions that omit the notebook. namespace, which instead only operate over individual cells. The latter are problematic for actions that need global context, e.g., our unused imports rules.

When the notebook.* actions are triggered, VS Code sends down the first cell (see: microsoft/vscode#193120) as the URI. This PR adds handlers for the actions and logic to use the notebook, rather than the cell.

Closes #320.

Test Plan

Used the following settings.json:

{
  "[python]": {
      "editor.defaultFormatter": "charliermarsh.ruff",
      "editor.codeActionsOnSave": {
        "source.organizeImports.ruff": true
      }
    },
    "notebook.codeActionsOnSave": {
      "notebook.source.fixAll": true
    }
}

Verified that imports used across cells were not removed (whereas "source.fixAll": true did cause imports to be removed).

[charliermarsh]

I'm pretty sure we only ever pass the full action here? This is for deferred actions.

[zanieb]

Sweet fix!