Remove CucumberReact, add CustomRendering

[davidjgoss]

🤔 What's changed?

Previously we had a root component <CucumberReact/> which was used to set some baseline styles and give a chance to inject custom rendering overrides.

We have managed to remove the need to set baseline styles, with defaults like font family and size being sensibly inherited from the parent page. So we can now replace this component with an optional <CustomRendering/> component just to handle that use case.

For consumers this is easier - you can use any component without needing to worry about whether you've slotted in the root one high enough.

For us, with all components having encapsulated styles we have more options for styling e.g. CSS-in-JS or whatever. We can also more easily use things like modals which tend to sit at a root-level in the DOM and escape cascading styles.

🏷️ What kind of change is this?

• 🏦 Refactoring/debt/DX (improvement to code design, tooling, etc. without changing behaviour)
• 💥 Breaking change (incompatible changes to the API)

📋 Checklist:

• I agree to respect and uphold the Cucumber Community Code of Conduct
• I've changed the behaviour of the code
  • I have added/updated tests to cover my changes.
• My change requires a change to the documentation.
  • I have updated the documentation accordingly.
• Users should know about my change
  • I have added an entry to the "Unreleased" section of the CHANGELOG, linking to this pull request.

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