Bundle extra Python packages as part of static assets output

[georgestagg]

So as to match the Wasm R package bundling now included in posit-dev/r-shinylive.

[georgestagg]

The maximum file size for GitHub Pages seems to be 100MB. There should be a default limit in place for packages larger than this.

c.f. posit-dev/r-shinylive#114

[wch]

For future reference, I think this would involve the following:

• Query PyPI (or other package repository, if specified) for packages specified in requirements.txt, along with their upstream dependencies.
• Download those packages.
• Add those packages to the pyodide-lock.json file, similar to how the shinylive_lock.json file is created and then merged into pyodide-lock.json.