NotebookCellTextModel leaks when closing a notebook

[jrieken]

• run OSS
• open and close .vscode/notebooks/api.github-issues (this sets the baseline)
• take a heap snapshot in DevTool > Memory
• open and close the notebook five more times
• take another heap snapshot
• show summary and filter by NotebookCellTextModel and notice each opening leaks 6 of those

I haven't fully understood the retainer path yet but it seems to stick around in some context of some ActionBar instance (which is likely also leaked). The root leakage might be something different but I guess these text models are the heavy parts