[cli] Support root-level Middleware file in vc dev (#7973)

Adds initial support for a root-level `middleware.js` / `middleware.ts` file in the `vercel dev` CLI command. This leverages the existing Edge Function invoking logic in `@vercel/node`'s `startDevServer()` function and applies the necessary response / rewrites / mutations to the HTTP request based on the result of the middleware invocation.

packages/cli/test/dev/fixtures/middleware-rewrite-query/middleware.js

@@ -0,0 +1,6 @@
+export default () =>
+  new Response(null, {
+    headers: {
+      'x-middleware-rewrite': '/api/fn?from-middleware=true',
+    },
+  });