Add some stuff to .prettierignore

Summary:
While testing D10143, I ran prettier --write . from the root and it printed some changes.

To avoid those changes (except for the one mentioned in D10143 that touches services/electron-update-server/index.js), I'm updating the .prettierignore here.

This should be a pretty minor change... mostly we run Prettier as part of ESLint, and follow .eslintignore for those.

Test Plan: prettier --write .

Reviewers: michal

Reviewed By: michal

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D10155