Setup fake web worker for jest

• Step 1: Setup fake web worker for jest
• Step 2: Fix Sourcegraph with MR diffs
• Upstream PR: Update GitLab domFunctions to use new interop

What does this MR do?

Webpack transforms _worker.js modules into nice consumable WebWorkers. Jest does not use Webpack so we don't get this out of the box.

• Previously: The old solution was to manually replace our workers with a stubbed out WebWorker.
• Now: Let's provide a fake implementation which is automatically applied to _worker.js modules. This greatly helps us with integration tests that are using web workers.

But why?

• It's pretty nice that the tests are now actually running these things. 👍
• It also enables us to write integration tests which are implicitly using theses (see next MR). 🎉

Does this MR meet the acceptance criteria?

Conformity

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  • I have included a changelog entry.
  • I have not included a changelog entry because no user-facing changes.
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