[chore] refactor exports

[jackyzha0]

Why

• Webpack doesn't treeshake classes very well. We also cluttered up the top-level namespace quite a bit by shoving everything in index.ts

What

• Split up the exports into semantically useful submodules
• Bumps @replit/river to 0.3.0

[bradymadden97]

Accepting to unblock, but I'd strongly recommend starting to add some doc comments to these exports, especially while you're already in here cleaning stuff up.

Normally when I start exploring a new library I start with the exports to learn how the pieces fit together. Since this library has a pretty large number of exports (more than I would expect tbh), I think it's even more important to have doc comments. I find doccomments to be much more useful than a separate document, as that usually quickly goes stale.

[jackyzha0]

Yup, I need to do another big refactor here that changes the types pretty significantly (WS-1403 and WS-684) so I was holding off on JSDoc but I can do that after this PR