Add tests (unit + e2e)

[tbrockman]

It's cool to have a working extension and all (especially one that's able to circumvent CSP), but in-order to ensure long-term successful maintenance and development of this extension we'll need to add some tests to verify we don't have any regressions in functionality.

Playwright is suitable for testing extensions e2e (check out my prune extension as an example).

Unit testing can be done with whatever Javascript test framework du jour that's most ergonomic and makes using TS painless.