Fix CI errors related to notebook cells

[pokey]

We currently have breakages in CI, eg https://github.com/cursorless-dev/cursorless/runs/6844372300?check_suite_focus=true. They are likely due to the new VSCode version changing the notebook API, which is probably breaking the following

cursorless/src/util/notebook.ts

Line 10 in 004ce14

export function getNotebookFromCellDocument(document: TextDocument) {