feat: page layer rewrite

[olliethedev]

• Removed _page_ layer type in favor of using any component type (like div, main, or custom components) as the root page layer. This enables use cases like building react-email templates.
• The componentRegistry is now passed as a prop to the UIBuilder component instead of being defined in a standalone file.
• Removed the script used to generate component schema definitions.
• Override panels of the UIBuilder component to allow customization.
• Moved core TS types to separate file to avoid confusion when using the component.