[monorepo prettier] Prettier should ignore yml files

Summary:
T3044

prettier should not auto lint yml files, this is handled separately

Test Plan: If you are using vscode, then saving a changed yml file will auto-apply prettier rules. With this change, that behavior should not happen. I'm not sure about other IDEs.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13655