feat(rsc): support export default { fetch } as server handler entry

[hi-ogawa]

Description

Previously rsc plugin supported only following server handler module:

export default function handler(request: Request): Promise<Response> { ... }

This PR adds support for a following export to align with well-known server runtimes such as

• cloudflare https://developers.cloudflare.com/workers/runtime-apis/handlers/fetch/
• vercel https://vercel.com/docs/functions/functions-api-reference?framework=other#fetch-web-standard

export default {
  fetch(request: Request): Promise<Response> { ... }
}

cc @markdalgleish