Multi-versioned web demo

[zaneduffield]

It would be nice if the web demo let the user select which version of pasfmt they wanted to use. With the latest release as the default, and options for each specific release as well as latest master.

Practically this would work by manually committing to the gh_pages branch in the web demo workflow (triggering on tags, and push to master), with one folder at the top level for each version we want to be accessible to the user. Then we could have pages at /latest and /master that redirect to other pages.

Then a drop-down could be added to the page when lets the user easily switch between versions.

I'm imagining that the entire demo (html, js, css, wasm) would be deployed under each versioned sub-directory; it's not worth the effort to make sure that the html and js is compatible with each version of the wasm library, and we may well want to make breaking changes.