Add basic e2e tests

[tobiasdiez]

Verify that the parsing of a couple of important websites works as expected. Testing that the import in JabRef works is probably not possible.

Reference: https://wxt.dev/guide/essentials/e2e-testing.html

[LyzardKing]

Can we not use the tests available in the translators to check this?

[tobiasdiez]

That would cover parts of it (and independently is a good idea). What I had more in mind with "e2e" is really remote controlling the browser: go to a website, click the extension button, verify that JabRef would be called with the correct info. That would test the whole setup.