feat: support webserver for speedscope cli

I'm working with callgrind files in a devcontainer. Unfortunately, this means that the "open in browser" behavior native to speedscope doesn't quite work "out of the box".

If possible, I'd love to see a little webserver built into speedscope so that I can run something like:

```
speedscope cachegrind.out.38776.gz --serve
Serving speedscope on http://localhost:46387
```

so that I can just quickly visualize the callgrind file.

Optionally, and I don't know how big an ask this is, it would be nice to extract all the various `Parser` -> `ProfileGroup` behaviors out of the react app into a separate lib so that I could potentially write this little server myself. In its current state, `speedscope` encapsulates the entire react app onto npm making it difficult to extend.

Aside, this project is awesome.