Core rewrites Next.js RSC streams #1340

One of the removals listed in #777.

What is wrong

crates/trusted-server-core/src/rsc_flight.rs rewrites the URLs inside a React Server Components Flight stream, which Next.js uses for its navigation responses and its inlined page data, and the publisher path calls it by media type. The Next.js crate (crates/framework/nextjs) carries the rest of that framework's handling, and this one piece stayed in core because it ran in the publisher path before a middleware could run on a media type other than HTML.

What changes

• The rewriter becomes a middleware of the Next.js crate. It runs where a [[fetch]] entry names it for the Flight media type, through the media types the stack added (A middleware can change HTML and nothing else, so a module has no place to change a text file #1277), and rewrites what it rewrites today.
• Core names no framework. The publisher path runs the entries the configuration lists and knows nothing of Flight.

Why

Every framework's code leaves core, and the stack gave a middleware a media type so that a change to a text response has a place. This is the last one that predates it.

What breaks

A deployment that fronts a Next.js site names the rewriter in a [[fetch]] entry for the Flight media type, as it names the rest of the framework's middleware. Without the entry, Flight responses pass through unrewritten.

Where it is

A series on the stack, as a draft on the fork, jwrosewell#35, on top of the source module series (fork pull request 34).