[feat] Steps (compiled addons etc)

[TheAlan404]

Most addons could be downloaded but some are compiled. This proposal (that will require a lot of code change) will allow there to be more complex addons.

The idea is that a Downloadable, instead of returning a ResolvedFile, will return Vec<Step>

pub enum Step {
  Download(ResolvedFile),
  Execute(...),
  Compile(...),
}