`devenv shell` remote environments in a flake-like way

Allow `devenv shell` to run remote environments in a flake-like way just like `nix run`, `nix develop` or `nix profile install` do:

Current `devenv shell` would be equivalent to `devenv shell .#`

Scenarios for remote repositories should be similar to `devenv shell github:repo/name`

This could also be further extended to other devenv commands like `build`, `processes`, `tasks`, etc.