[Nix] Prompt user to install direnv

Summary:
Prompt the user to install direnv through homebrew.
Sets up zsh hook if missing, provides link for other shells.

Depends on D6313

https://linear.app/comm/issue/ENG-2753

Test Plan:

# if direnv is already installed, uninstall
brew uninstall direnv

nix develop
# should get prompted

Reviewers: atul, varun, bartek, O1 Nix

Reviewed By: atul, varun, bartek, O1 Nix

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D6315