First local run: add supabase to instructions & improve shebang

[tennox]

What I had to change in order to run locally:

• start supabase (was not part of CONTRIBUTING.md instructions)
• modify supabase.sh to
  • initialize .env.local
  • use a more generic shebang that supports OSes where bash is not in the default location (NixOS in my case)

[vercel]

Someone is attempting to deploy a commit to the TypeCell Team on Vercel.

A member of the Team first needs to authorize it.

[YousefED]

Thanks! Sorry for the delayed replies due to xmas break

[pietgk]

Tried this PR, and it helped me to get local development started!
Thx for the PR 👍