[WIP] Support standalone nixvim

[pta2002]

This represents a major rearchitecture for nixvim, so I'm leaving this up to track the progress for now, and to serve as a reference for any breaking changes during transition.

The main change is, of course, being able to use nixvim standalone. To do this, you should use the new build function, which takes in two arguments: the system architecture (e.g. x86_64-linux) and the configuration. For the new configuration, do not use the programs.nixvim. prefix.

For module development, the main change is that you should no longer prefix your modules with programs.nixvim..

Checklist:

• [x] support build function
• [ ] support home-manager
• [ ] support raw NixOS modules
• [ ] update documentation and README to reflect changes

P.S: Addresses #16

[pta2002]

For some reason, I can't seem to get the NixOS and home-manager modules working - they build the package, but it does not seem to read any config.