Introduce monorepo tooling

[johnste]

The current repository is pretty messy, with the config-api, the go/objc app and the debug window contents spread out in a custom hierarchy. As I'd like to expand the Finicky project to also include browser extensions, and maybe apps for other platforms, it makes sense to create a sensible structure for this, one that understand dependencies between them.

I'm mostly familiar with NX, but I'm not sure which tooling (if we need any at all) suits the project's needs.