JS code is now auto-formatted with Prettier

Dear fellow contributors,

if you are contributing to Flarum's JavaScript code, please be aware that we are now using Prettier to auto-format our JS files.

How will this affect you?

Local development: please ensure your JS dependencies are up-to-date (npm install). This should automatically set up Git hooks using husky, which will run Prettier before you make a commit. (If it doesn't seem to work, please make sure your Git and Node versions fulfill husky's minimum requirements.)
Contributing: When sending a PR to Flarum, the builds will fail if code has not been formatted locally. To do so, set up Git hooks or run npm run format locally and commit the results.
Existing PRs with JS changes: Those probably now have conflicts, but they should be easy to resolve. I will go through them all and post instructions for resolution, if necessary.

Thanks for your attention! 🎤

If one of your merge request encounters conflicts because of the formatting changes, you should be able to resolve those using the following steps:

Make sure your master branch is up-to-date
Back up the current branch by running git branch <name>-backup (e.g. git branch patch-1-backup)
On the master branch, run npm install
Switch back to your branch, and rebase on the latest master by running git rebase master
When you encounter conflicts, run git checkout --theirs -- js/path/to/the/conflicted/file for each file that is shown as having conflicts
Execute npm run format in the js/ directory
git add . and git rebase --continue
If necessary, jump back to step 4.
Force-push using git push -f to update your merge request. 😃