Specific Env Vars and Edit Command

[Burtannia]

Description

Specific Env Vars

1. Introduce two new environment variables / arguments: NH_OS_FLAKE and NH_HOME_FLAKE to set the flake for the NixOS and Home Manager configs respectively.
2. Remove the FLAKE argument since it's no longer needed and, as others have pointed out FLAKE is not a very specific env var #88, it's not very specific. Note that a fallback is in place to FLAKE if NH_..._FLAKE is not set so as to make the change a bit more backwards compatible.
3. Fallback to current directory if neither NH_..._FLAKE nor FLAKE is set.

Edit Command

Running nh <os | home> edit uses the aforementioned env vars to open the default editor (as specified by EDITOR) in the flake directory.

[viperML]

I will implement this in #151