Quarto notebook exporter (Positron)

[seeM]

This PR sets up a "notebook exporter" for Quarto when inside Positron, which reuses the same implementation as the quarto.convertToQmd command (i.e. the quarto convert CLI). The quarto.convertToQmd command is also hidden when the exporter is registered (i.e. in Positron).

Until posit-dev/positron#14152 is merged, you'll have to test against that branch. Should have no effect in VS Code.

Demo:

Screen.Recording.2026-06-10.at.17.29.06.mov

I also added the outline of a test, but can't yet run it until we're able to test against Positron.

Related to posit-dev/positron#9791.

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[vezwork]

I glanced over the code and it looks good. 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 do a closer review, test, add changelog, and merge later.

[seeM]

@vezwork cool! In my testing, it works in Positron (on this branch) and doesn't break anything in VSCode. I'll go ahead and merge, but please let me know if you find any issues and I'll follow up with a fix.

[vezwork]

Tested the merged change in VSCode and there's no issues. If we run into any, we could consider putting activateNotebookExport behind a isPositron if statement.