This is split from T261857: Implement bracket matching in CodeMirror behind a feature flag. The idea is to at least try to write a browser test. A single "smoke test" for each editor is enough, i.e. a single test that warns us when bracket matching stops working for some reason.
- Copy-paste the basic setup for browser tests from an extension that already has browser tests (e.g. from Two-Column-Edit-Conflict-Merge)
- Make sure the browser tests run on our CI
- Fake the feature flag when the browser test runs
- Write a test that starts editing a page, positions the cursor, and check if the relevant DOM nodes for the highlighted brackets appear somewhere on the page
- Test both the 2010 and 2017 (VE) wikitext source editors.