Cleanup FR API for Release

[adamraine]

In https://github.com/GoogleChrome/lighthouse/issues/14048, we are going to expose the FR api on the default node entry point. Any breaking changes to the FR API should be done in a breaking release after this point. Before we do that, there are a few cleanup changes I would like us to consider:

• [x] Remove configContext and just use flags instead
• [ ] Move user flow types to LH.UserFlow
• [ ] stepName -> name (This is what we use in the flow artifacts json)
• [ ] export FR methods on core/index exports #14048

This list isn't final, the goal here is to get the API into a state that we are happy with before committing to it for at least one breaking release cycle.