feat: handle subshell of subshells

[eureka-cpu]

One thing I've always hated is that I forget I'm in a nix shell and then navigate somewhere and enter a nix shell. It would be cool if we could detect if the subshell is a nix shell and if it is we just exit the subshell prior to entering a new one. Not sure if it's feasible but I'd definitely benefit from the feature.

[eureka-cpu]

Turns out this may be easier than expected, we basically just need to always use the --pure flag for nix-shell and I think for nix develop and nix shell the ignore-env option looks promising. The difference between this and the original idea is that intead of exiting previous subshells, the new subshell has enforced purity by default.