next.js

a5a47b2a - fix: throw error during build when invalid export for Proxy (#84886)

fix: throw error during build when invalid export for Proxy (#84886) ### Why? When Proxy/Middleware had invalid exports (valid: named function `proxy` for `proxy.js`, named function `middleware` for `middleware.js`, or default function), it errored during runtime but not during build. This error can sneak into production as it only throws during runtime. ### How? Walk AST of the Proxy/Middleware file, and if a valid export is not found, throw during build. --------- Co-authored-by: Benjamin Woodruff <benjamin.woodruff@vercel.com>