chore(ci): add direnv virtual env integration

[flrgh]

This adds an .envrc template to integrate local development with direnv workflows. I have been using a similar workflow with a self-maintained .envrc file for some time, and it works great!

It's not fully 1-1 with how the bash/fish env files work due to direnv not supporting exported functions, but I think it's fine for now. This could be solved by refactoring some things in ./scripts/dependency_services, but I want to keep the PR small and simple.