Flag, report document selectors that use notebookType and cell-scheme

[jrieken]

Testing #194077

While working on GHINB joh/civic-tahr I was wondering why my code action provider wasn't ever called. Turns out I had the following selector configured

{ scheme: 'vscode-notebook-cell', notebookType: 'github-issues' }

This never matches because the notebook-types makes us use the scheme of the notebook file, not its cells. This behaviour is clearly documented (by me 🙊) but I still missed it... We should be nice and somewhere print a clear warning when this happens