Support outline with notebooks

Issue Type: <b>Bug</b>

Using a GH Issues notebook.

1) Open a notebook
1) Cmd+Shift+O => Quick outline appears
3) outline view remains empty


VS Code version: Code - Insiders 1.46.0-insider (da3bf754c3c0eabda1dc80ed9d627679ee560697, 2020-05-19T10:04:14.981Z)
OS version: Darwin x64 18.7.0
Remote OS version: Linux x64 5.3.0-1020-azure