Add warning to the import site

Author: devjiwonchoi

packages/next/config.js

@@ -1 +1,12 @@
-module.exports = require('./dist/shared/lib/runtime-config.external')
+let hasWarned = false
+
+module.exports = (() => {
+  if (!hasWarned) {
+    console.warn(
+      // ANSI code aligns with Next.js warning style from picocolors.
+      ' \x1b[33m\x1b[1m⚠\x1b[22m\x1b[39m Runtime config is deprecated and will be removed in Next.js 16. Please remove the usage of "next/config" from your project.'
+    )
+    hasWarned = true
+  }
+  return require('./dist/shared/lib/runtime-config.external')
+})()