Allow Butterchurn to be used by consumers

#729 built out a nice API for allowing consumers to add Butterchurn. This will be useful for:

* https://desktop.webamp.org/
* https://archive.org/details/milkdrops
* And other users (#739)

## Here's what we still need to do:

- [ ] Make Butterchurn methods public
- [ ] Make window size/position option public
- [ ] Add Butterchurn options types to `index.d.ts`
- [ ] Add Butterchurn options to `docs/usage.md`
- [x] Add an example project in `examples` which uses Butterchurn
- [ ] Publish a new version to NPM

## Nice to haves

Transferred from #729

- [ ] Download `.milk` presets from URL
- [ ] Avoid bug where the song title is shown a second time when changing into/out of desktop mode (also affects first preset transition being immediate)
- [ ] Figure out why fullscreen transition is not as nice as it was before. Was screenful doing something for us?
- [ ] Load preset pack along with the preset specified in the URL
- [x] Handle non .milk/.json when loading full directory (currently errors out, without loading rest of dir)
- [x] Allow loading presetMapUrl, not just single presets
- [x] Prevent preset transition on initial preset load (currently blending with default "empty" preset)