[nix] Introduce .envrc to invoke Nix via direnv

Summary:
So Nix stuff happens automatically when we cd into repo instead of having to run nix develop a million times.

(I installed .direnv via brew install direnv, but ideally it would be installed via Nix?)

Test Plan:
cd comm

cd comm/native

Reviewers: jon, ashoat

Reviewed By: jon, ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D6152