Introduce WebKitHeadless browser testing, using the karma-webkit-launcher plugin. The plugin uses playwright to launch it's WebkitHeadless browser, and uses that browser to run karma tests on.

This is significantly better than the proposed solution in #8396, where we forked the old karma-safari-launcher plugin, because it uses a headless browser. This means that

• We don't need to manually enable Full Disk Access on Safari to the host process
• The headless browser process will exit after tests are complete
• There are no browser popups when running the tests, since there's no head to pop up

How to run locally:

yarn install # make sure the new packages are installed
npx playwright install webkit # install playwright's webkit browser
BROWSERS='WebkitHeadless' yarn test:browser # or any other command that launches a browser for karma tests

Testing:

I ran all of the browser tests against WebkitHeadless locally, and did not encounter any unexpected failures (the flakes in firestore still occur). Everything works as expected.

When running the same tests in CI, I noticed that several packages are now flaky (seemingly only in CI?): installations, storage-compat, database, database-compat, app-check. It's not clear why these are suddenly only failing in the headless webkit browser in CI, so let's skip them for now, and add a TODO to fix this later.