Rearchitect as a plugin system

[breezykermo]

I have been thinking that it would be useful to have a way to produce a more refined HTML output that embeds revealjs, and thus allows an alternative to touying or other frameworks which produces web-native presentations/slides.

Rheo is really conceived as a way of customizing the way that Typst compiles documents, where EPUB and HTML are the two first targets. It wouldn't be too difficult to generalize Rheo a bit such that it supports a module system wherein new targets could be additively supported, i.e.:

rheo compile project_uno --slides

The support for the new plugin could, perhaps, be specified in a projects rheo.toml:

[[plugins]]
name = "revealjs"
flag = "--slides"
code = "./rheo_revealjs"

To support this, we would first need to rearchitect the codebase so that new outputs are possible in principle. We would also need to think through a proper design for the plugin structure.