feat(flake): Add support for using icedos/core as a Flake, instead of needing clone

[jim3692]

Tasks:

• Expose as an importable library
• Ability to build
• Track configuration location for compatibility with icedos rebuild
• Keep separate flake.lock for NixOS
• Backwards compatibility with current (clone-based) configurations
• Integration with icedos update, to update the icedos input of the Flake
• Update readme

Example flake.nix:

{
  inputs.icedos.url = "github:IceDOS/core";

  outputs =
    { icedos, self, ... }:
    icedos.lib.mkIceDOS {
      configRoot = self;
    };
}

More details:

• The user's config.toml must be on the same path as the user's flake.nix
• The user may add an extra-modules directory in the flake.nix's path, that behaves like the current .private
• The icedos/core input must be named icedos