Add prettier to `CONTRIBUTING.md`

[Maciej Domanski]

Add info on how to simply install Prettier (e.g., brew install prettier) and how to use it to avoid Run Prettier to fix workflow errors.

[mschwager]

Nice catch, I'd suggest we add it here: https://github.com/trailofbits/semgrep-rules/blob/main/CONTRIBUTING.md#linting