Supporting `fontc` as an experimental alternative compiler

I would like to start testing using `fontc` in place of `fontmake` in the gftools builder pipeline, and I'm hoping to discuss how best to do this. Ideally this work would be isolated and non-invasive enough that it could happen in this repo, instead of in a fork.

The main requirement for this work would be some way to pass a flag that would opt us into `fontc` mode. I imagine that this would involve adding some new 'Operations' that can be run in place of the current `BuildOTF` and `BuildVariable` classes, and which would be backed by `fontc`. (An environment variable could be used to pass a path to the `fontc` executable.)

When the user passed the `--experimental-fontc` (or whatever) flag, we would then use the alternative operations instead of the default ones; I'm not sure how best to plumb this through, but I could imagine possibly modifying the `known_operations` or else adding an `operation_overrides` dict or something like that; I imagine this project's maintainers might have a better sense of how best to do this.

Does this make approximate sense? Is there anything else I'm missing?