This adds the ability to configure an application via an env.config.js file rather than environment variables or .env files.  This mechanism is much more flexible and powerful than environment variables as described in the associated ADR.

It also improves documentation around how to configure applications, providing more detail on the various methods.

Finally, it allows the logging, analytics, and auth services to be configured via the configuration document now that we can supply an alternate implementation in an env.config.js file.  This allows configuration of these services without forking the MFE.

fixing a few typos, adding a section about the relationship between JS file config and runtime config, and cleaning up some formatting.

Based on PR feedback - be clearer about how the configuration methods overlap and what their best use cases are.

Turns out if you put each test in its own file, jest.mock can be used to change the implementation of env.config.js for each test file.  This lets us test all the clauses in jsFileConfig.  It also means we no longer need the actual env.config.js file, since we’re mocking it!

Previous commit removed this file, forgetting that it’s also necessary for the example app.  Reinstated it here with a value specific to the example app so we can still prove the test suite works.

We have agreed we’d like to DEPR environment variable config, so I’m going to merge the ADR as “Accepted”