fix: enable preview commands in any ipynb editor

[seeM]

Enables "Preview" and "Preview Format" commands in any .ipynb editor, since they do work in that case. Fixes posit-dev/positron#9792.

Here's what it looks like in the Positron Notebook Editor:

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[vezwork]

Haven't tested yet, but if you've tried it in both Vscode and Positron, and after you add a changelog entry, please go ahead and merge (if you have permissions to). Otherwise I'll test, add changelog, and merge later.

[vezwork]

I tested this on main after it was merged. I downloaded the ipynb from https://jupyter.org/try-jupyter/lab/ and opened it in the notebook editor in vscode. I then clicked the three dots in the upper right and clicked Preview. I got an info toast "No Quarto document available to render" and nothing else happened. I got the same result when I clicked Preview Format. Notably, convert to .qmd does work fine.