Summary

Refactors the Setup Wizard steps to use named identifiers instead of numeric indices - for example, start, configuration and finish. Future PR's will rely on this when optionally adding steps - for example, if third party form plugins are detected, the onboarding Setup Wizard might add an additional step for the user to automatically replace third party forms with Kit forms.

Testing

Existing tests pass.

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)