Feat/astro virtual modules

[Tate-CC]

This PR:

• Adds support for the astro:content and astro:assets virtual modules in Bookshop components (Support astro:content inside Astro components in the Visual Editor #154)
• Adds support for Astro slots inside Bookshop components (using Astro.slots.render() breaks live editing #178)
• Astro Bookshop will now try to use your configured vite plugins as a fallback during build (error: No loader is configured for ".yml" files #179)