[Direnv] Use nix-direnv if possible

Summary:
nix-direnv has better amoritorized costs for initiating a shell.

Part of https://linear.app/comm/issue/ENG-2751

Test Plan:

Install direnv, script being added in later diff

direnv allow .
cd ..
cd comm
# might be slow on first execution

cd ..
cd comm
# all following executions of direnv should be relatively fast

Reviewers: atul, ashoat, varun

Reviewed By: atul, ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D6311