Improve playground versioning

[irh]

Currently the playground is a yew application, with snapshots made for published versions and placed in static/play-0.xx. The docs snapshots refer to the specific URL.

I think it would be much cleaner to have a single playground URL, with version selection via a URL parameter. The first step will be figuring out how to dynamically load the relevant .wasm, which might involve splitting out the versioned koto runtime from the non-versioned playground.