Need to modularize darwin config folder

[HyunggyuJang]

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Currently, I'm managing macOS system configuration in one big file, namely configuration.nix.
It was mandatory at first, when the code's footprint is small enough, thus, it is more reasonable to manage it in one file.

However, become now, my configuration files starts to be too ambitious so that other individual modules like home-manager engrossed within it, as a result, the time it consumes to reflect tiny patches triggers whole process, notably logically irrelevant part also get evaluated and refreshed along with.

I might have to decompose home-manager from nix-darwin, since the inherit characteristics of each are quite different; and it will effectively reduce the time and effort to manage my own configuration files.

Other than that, I need to adopt modules file system as that of nix-darwin itself. I have to search through whether I can override the default modules as that of Doom Emacs system.

I know that the author of Doom Emacs admires and enjoys nix's philosophy, it is very likely nix-darwin also supports that feature.