Standardize env exports such that they work in zsh and in fish

[zph]

I've doggedly held onto my zsh configs and I want to keep backwards compatibility...

What I'm thinking is that I have a generic ~/.config/shell/env.d/* folder that contains only env files and then I can ensure those load correctly from both zsh and from fish. On the fish side I can extract them using something like https://github.com/oh-my-fish/plugin-foreign-env/blob/master/functions/fenv.main.fish