Packaging nari for PyPi

[ayyaruq]

We currently provide a basic nari package, but with the ACT reader being effectively standalone and future native extensions and other readers/writers being on the horizon, we need to think about how to structure it.

I'm proposing using setuptools extras_require so we can do things like pip install nari[act] and get nari along with the ACT reader. For native extensions, they should be bundled into whatever their module is as part of the module, rather than an explicit ext suffix. This could be used with namespace packages to additionally avoid needing to split ACT support out into a separate repo, but may require better separation of the code so that readers are more structurally independent.

This design is open to anyone for comment/feedback, and is being done here rather than in Discord so we have a better record and can deal with the main contributors being spread over 3 timezones with little overlap.