Allow webpack config, but warn if no turbo config (#49170)

Allows webpack config so Turbopack can be used for dev and webpack can
be used for production. Also warns if there is no corresponding
Turbopack config (only a warning since webpack config could be prod
specific)

Closes WEB-984

<img width="647" alt="Screenshot 2023-05-03 at 12 32 08 PM"
src="https://user-images.githubusercontent.com/2865858/236027314-4c762b4d-ebe4-486a-bd64-5215dcf60db6.png">

packages/next/src/lib/turbopack-warning.ts

@@ -25,6 +25,7 @@ const supportedTurbopackNextConfigOptions = [
   'experimental.swcFileReading',
   'experimental.forceSwcTransforms',
   // options below are not really supported, but ignored
+  'webpack',
   'devIndicators',
   'onDemandEntries',
   'experimental.cpus',
@@ -98,6 +99,9 @@ export async function validateTurboNextConfig({
   let babelrc = await getBabelConfigFile(dir)
   if (babelrc) babelrc = path.basename(babelrc)
 
+  let hasWebpack = false
+  let hasTurbo = false
+
   let unsupportedConfig: string[] = []
   let rawNextConfig: NextConfig = {}
 
@@ -156,6 +160,13 @@ export async function validateTurboNextConfig({
       : supportedTurbopackNextConfigOptions
 
     for (const key of customKeys) {
+      if (key.startsWith('webpack')) {
+        hasWebpack = true
+      }
+      if (key.startsWith('experimental.turbo')) {
+        hasTurbo = true
+      }
+
       let isSupported =
         supportedKeys.some((supportedKey) => key.startsWith(supportedKey)) ||
         getDeepValue(rawNextConfig, key) === getDeepValue(defaultConfig, key)
@@ -185,6 +196,17 @@ export async function validateTurboNextConfig({
     feedbackMessage = chalk.dim(feedbackMessage)
   }
 
+  if (hasWebpack && !hasTurbo) {
+    console.warn(
+      `\n${chalk.yellow(
+        'Warning:'
+      )} Webpack is configured while Turbopack is not, which may cause problems.\n
+  ${chalk.dim(
+    `See instructions if you need to configure Turbopack:\n  https://turbo.build/pack/docs/features/customizing-turbopack\n`
+  )}`
+    )
+  }
+
   if (babelrc) {
     unsupportedParts += `\n- Babel detected (${chalk.cyan(
       babelrc