feat: add support for non-flakes

[RaitoBezarius]

Via flake-compat.

[astro]

Do you really need this?

[RaitoBezarius]

I'm not sure there's an easy way to get access to the package set exposed, short of rewriting the flake.nix, right?

[RaitoBezarius]

Happy to drop flake-compat if you prefer me to re-expose it via default.nix and re-import in the flake.nix directly.

[astro]

Yes, I'd prefer one dependency less in flake.lock for the case where flakes are not used :)

[RaitoBezarius]

Okay, will send a more complicated PR for that, then :).

[astro]

Let me know if you encounter problems because if the other way becomes too many lines of code we should resort to the flake-compat solution.