Improvements to API Routing

[blittle]

Improvements to API Routing:

• Create predefined error response classes: Fix the killing of processes in monorepo hydrogen#450 (comment)
• Automatically cast string responses to new Response
• Automatically cast object responses to new Response(json.stringify(obj)) with JSON content type.
• Allow .index.sever.js components in folders
• Consider changing the default route structure to be "routes" instead of "pages"
• Support multipart formData - see Implement multipart parsing nodejs/undici#974
• Support for data routes with JS disabled in the browser
• Provide a useShopQuery helper as an argument #779

[frehner]

Allow /comments/index.server.js and /comments/[handle].server.js folder structure? Shopify/hydrogen#391 (comment)

[blittle]

Allow /comments/index.server.js and /comments/[handle].server.js folder structure? #391 (comment)

Probably more a general improvement to routing, this could help non API routes as well. Happy to address it here at the same time though!