quarto format support

[githubpsyche]

Quarto is a scientific and technical publishing system built on Pandoc. It's been growing in popularity lately, now has its own vscode extension, and has received support in recent updates to jupytext. If you have quarto installed, Jupytext lets you edit .qmd documents as notebooks in Jupyter, and pair .ipynb notebooks with .qmd notebooks.

The current version of vscode-jupytext doesn't support the quarto format, but a small change to packages.json's `"contributes" / "menus" / "explorer/context" list adds support for the format. We just add:

                {
                    "command": "jupyter.openAsPairedNotebook",
                    "when": "resourceLangId == quarto"
                }

We also have to update the extension's jupytext version to one with quarto file format. I confirmed 1.14.1 works.

And finally, to allow notebooks instantiated by vscode-jupytext by this command to update the source qmd file, we probably still need to address #15 with a change like the one I suggested.

Interoperation with the quarto extension doesn't seem perfect though.