This update introduces a new system for modifying website design elements, specifically shapes within images and backgrounds. It allows plugins to directly adjust these elements, providing greater control and customization options for website builders. This improves the ability to tailor website designs to specific business needs.

This commit defines `image_shape_groups_providers` and
`background_shape_groups_providers` as new plugin resources.
These resources return functions that receive the current shape groups,
allowing plugins to mutate them directly or return groups to be merged
into the final configuration.