Feedstocks with scripts in activate.d/deactivate.d should have scripts for each supported shell

Issue: There are lots of supported shells in conda, and each sources a different script extension in /etc/activate.d and /etc/deactivate.d

So any feedstock that sets environment variables in only .sh or .bat script will fail if the user is using powershell, fish, zsh, csh etc.

If this issue is resolved (https://github.com/conda/conda/issues/6820) this may not be necessary, but for now it is a huge pain if one wants to use powershell.