Add support for embedding host application into website

[hannobraun]

It should be possible embed the host application into a website, so Fornjot models can be viewed there. This includes compiling the host application to the wasm32-unknown-unknown target, and making sure the resulting binary works when embedded in a website.

Whether the current host application can be made to work that way is an open question. It might be necessary to create a second web-only host app. If that were to happen, those two hosts should share most of their code. This would require breaking up the current host into smaller libraries.

This issue is related to #71, which needs to be implemented for web support to come to fruition. However, these two issues are somewhat independent, in that a pre-loaded example model can be used to get web support to work. Being able to load arbitrary WASM models can be a separate step.

[hannobraun]

I've decided to replace this issue with more specific and actionable ones. Closing in favor of #815 and #816.