Add new port shells/envy

Envy is a shell helper program that automatically sets environment
variables when you enter certain directories, and properly undoes
its actions when you leave them.

Envy can be used to replace the shell integration part of tools like
pyenv, nvm and asdf. It sets environment variables with the root of
the current git repository and the active branch without executing
git, which you can use to customize your shell prompt. It also loads
custom environment variables from local .envy files.