nixshell: implement `nix shell` support

[MarcelCoding]

related: #355

Adds support for indicating presence of nix-shell, nix shell and nix develop.

nix-shell impure:

nix-shell pure:

nix shell:

nix develop:

Acceptance Checks

• [x] Documentation is up-to-date:
  • [ ] Add entry in feature list of README.md ;
  • [ ] Add entry in features' overview in docs/ ;
  • [ ] Add section in feature list to document
    • [ ] Features' flag ;
    • [ ] Prompt symbol ;
• [ ] Default are defined in conf.d/pure.fish for:
  • [ ] Feature flag ;
  • [ ] Symbol ;
• [x] Tests are passing (I can help you :hugs: ):
  • [ ] Config are tested (cf. tests/_pure.test.fish) ;
  • [x] Feature is tested in tests/feature_name.test.fish ;
• [ ] Customization is available ;
• [x] Feature is implemented.

[edouard-lopez]

Please edit your commit message to follow the commit conventions as we use it to manage the project version

[edouard-lopez]

I'm closing this as there is no activity and the feature's behaviour is not clearly tested. Please provide your insights if you are interested by this feature.