Upgrade to Theme Customizations Instead of Child Themes

[arunshenoy99]

Proposed changes

Story: https://jira.newfold.com/browse/PRESS0-1919

This moves the DIY flow away from child theme generation to using theme customizations by doing the following:

1. Using custom style variations for theme.json customizations.
2. Using template part customization for updating template parts.
3. Programmatically deselecting inactive fonts so that they are not written into the DOM during rendering.
4. Keeping YITH Wonder as the active theme.
5. Completely removing child theme generation.

Tests are failing as we need: newfold-labs/wp-module-onboarding-data#111

Type of Change

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

Video

Checklist

• I have read the CONTRIBUTING doc
• I have viewed my change in a web-browser
• Linting and tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments

[officiallygod]

Looks Good