fix: handling http response headers

[lazarv]

Moves route matching and route component loading into middleware when using the file-system based router to remove async implementation in the React Server Component entrypoint of the router.

Changes RSC payload handling to use the same approach as in the HTML SSR renderer to handle async components better. Introduces a rendering lock mechanism to suspend sending the response to be able to handle response headers in async components until the lock is released.

Refactors HTTP response header handling to use Headers directly and introduces helper functions to set, append, delete, or clear response headers in the HTTP context.

Fixes an issue with accessing the Response sent to the client when using useResponse in an async React Server Component.

Adds test cases for above changes and updates the docs to include the new helpers and to give more details about how to handle response headers.

#100

[github-actions]

✅ Documentation preview deployed to: https://react-server-docs-mxyyw1ryq-lazarv-team.vercel.app