flip the default for custom servers

Author: lukesandberg

packages/next/src/server/next.ts

@@ -534,13 +534,20 @@ function createServer(
   options: NextServerOptions & {
     turbo?: boolean
     turbopack?: boolean
+    webpack?: boolean
   }
 ): NextWrapperServer {
   if (
     options &&
     (options.turbo || options.turbopack || process.env.IS_TURBOPACK_TEST)
   ) {
+    if (options.webpack) {
+      throw new Error('Pass either webpack or turbopack, not both.')
+    }
     process.env.TURBOPACK = '1'
+  } else if (!options.webpack) {
+    // If no options are set we default to turbopack
+    process.env.TURBOPACK = 'auto'
   }
   // The package is used as a TypeScript plugin.
   if (