Bug 1830265 - Tidy up .prettierignore. r=Gijs,sync-reviewers,markh

Enables Prettier for (these don't need to be excluded):
- extensions/pref/autoconfig/test/unit/autoconfig.js
- services/sync/tests/unit/prefs_test_prefs_store.js

Removes duplications where the files are already ignored in the .eslintignore section.

Re-organises the structure of the remaining parts to make it clearer why they are being excluded.

Differential Revision: https://phabricator.services.mozilla.com/D176636