Fix syntax error that broke fish support (#36)

Add support for the 'nix develop' command (#34)

Support `nix run` and drop support for `nix shell` (#13)

`nix shell` didn't exist from Nix 2.0 to 2.3. And in 2.4 it keeps
the shell, anyways, so any-nix-shell is not required.

Fixes #10

Restore stable Nix compatibility (#8)

Stable Nix doesn't have nix shell, which means the most recent update
broke any-nix-shell for people using the defaults.

Here, I've added a simple version check to restore the old behaviour
on Nix versions that don't have nix shell.

Merge pull request #6 from nrdxp/fix-nix-run

`nix run` is now `nix shell`

nix-shell -> environment

Set SHELL variable while in nix-shell