Bookmarklet for marking GitHub PR

When run on a GitHub PR, this bookmarklet will mark all `vendor/*` files as _✅ Viewed_ and hide them.

```javascript
javascript:document.querySelectorAll('.js-file').forEach((f) => { h = f.querySelector('.file-header'); cb = f.querySelector('.js-reviewed-checkbox'); if(h.attributes['data-path'].textContent.startsWith('vendor/')) { if(!cb.checked) cb.click(); f.style.display = 'none'; }})
```

Via @rgatti

Start with the JavaScript bookmarklet shown in the issue, then inspect the repository layout for an existing location for scripts or usage notes. Done should make the bookmarklet available for use on a GitHub PR, where it marks vendor/* files as viewed and hides them.

Written by the indexing model from the issue text.