Self contained Python Data Loaders with `uv run` scripts

[metafeather]

Python self-contained UV scripts that define inline dependencies avoid the need for a .venv to be compatible across Data Loaders and it would be great for them to have first-class support

(Also suggested for Observable Framework: observablehq/framework#2038)

[mbostock]

So instead of this:

notebook-kit/src/interpreters/index.ts

Line 22 in 20a50c2

return ["python3", []];

We’d use uv run. That’s pretty easy. But we have to figure out whether we would require uv, use it if available, or have users opt-in via config or a different cell type.