Indicate loading progress of `fetch`able entities

As a quality-of-life improvement for people on slow connections, it would be nice to expose the loading progress of (e.g.) the Pyodide WASM bundle in the iodide console. (Since this cuts across two projects, I didn't feel comfortable hacking on it myself.)

I'm imagining something like this:

Start by tracing how fetchable entities, including the Pyodide WASM bundle, are loaded in the iodide console and where that loading crosses into the other project. Define the progress information and integration boundary before implementing it; done means users can see loading progress on slow connections.

Written by the indexing model from the issue text.