Setup Danger (#6994)

This PR sets up [Danger](https://danger.systems/js/) to help us codify
some of our PR rules.

As an initial rule, Danger will check to ensure that every PR has a
`Release Notes` section:

<img width="943" alt="Screenshot 2024-01-29 at 11 50 12 AM"
src="https://github.com/zed-industries/zed/assets/1486634/4d56e759-e72f-4bc0-8e74-42c55e2e6888">

Release Notes:

- N/A