Allow progress reporting when loading runtime dependencies

https://github.com/emscripten-core/emscripten/blob/9e4fcf9c87522642719c9ec75df47363561ca6ea/src/library.js#L3618-L3632

In the above code there is an onload and onerror, I would like to support an `onprogress` there too.

Currently it's possible to somewhat monitor dependencies download through `monitorRunDependencies` in the Emscripten Generated HTML, you can modify the function this Module component uses to get at least intermediary report per dependency. My wasm code though is around 6MB, it's usually fast to download but occasionally I get into a region where my phone has poor connection and it takes a bit, but I get no progress report.

I know every bit of code added impacts codesize of everyone, so I don't have any current suggestion on how to do so.

With onprogress event, people could then report download progress with something like:

```JavaScript
if (e.lengthComputable) {
setStatus('loading ' + Math.round((e.loaded / e.total) * 100) + '% ...');
}
```