Switch to postcss.config.mjs, add type (#63380)

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### What?

Now that @phanect added support for ESM PostCSS config files in PR
#63109 (original issue #34448), PostCSS can use ESM config by default.

It needs to use an `.mjs` extension by default because `create-next-app`
scaffolds CommonJS apps by default.

This will also work with ESM projects which have added `"type":
"module"` in their `package.json`

### Why?

1. To convert one more file to ESM
2. To use the modern format
3. To follow other similar migrations that have taken place in the
Next.js codebase (eg. `next.config.mjs`)

### How?

- Change file extensions from `.cjs` to `.mjs` (change similar to PR
#58380)
- Change module format from CommonJS to ESM
- Add type for the config, for users who enable `checkJs: true` in
`tsconfig.json`

Co-authored-by: Sam Ko <sam@vercel.com>

packages/create-next-app/templates/app-tw/js/postcss.config.mjs

@@ -0,0 +1,8 @@
+/** @type {import('postcss-load-config').Config} */
+const config = {
+  plugins: {
+    tailwindcss: {},
+  },
+};
+
+export default config;

packages/create-next-app/templates/app-tw/ts/postcss.config.mjs

@@ -0,0 +1,8 @@
+/** @type {import('postcss-load-config').Config} */
+const config = {
+  plugins: {
+    tailwindcss: {},
+  },
+};
+
+export default config;

packages/create-next-app/templates/default-tw/js/postcss.config.mjs

@@ -0,0 +1,8 @@
+/** @type {import('postcss-load-config').Config} */
+const config = {
+  plugins: {
+    tailwindcss: {},
+  },
+};
+
+export default config;

packages/create-next-app/templates/default-tw/ts/postcss.config.mjs

@@ -0,0 +1,8 @@
+/** @type {import('postcss-load-config').Config} */
+const config = {
+  plugins: {
+    tailwindcss: {},
+  },
+};
+
+export default config;

packages/create-next-app/templates/index.ts

@@ -51,7 +51,7 @@ export const installTemplate = async ({
   if (!tailwind)
     copySource.push(
       mode == "ts" ? "tailwind.config.ts" : "!tailwind.config.js",
-      "!postcss.config.cjs",
+      "!postcss.config.mjs",
     );
 
   await copy(copySource, root, {

test/integration/create-next-app/lib/specification.ts

@@ -68,7 +68,7 @@ export const projectSpecification: ProjectSpecification = {
         'pages/_app.js',
         'pages/api/hello.js',
         'pages/index.js',
-        'postcss.config.cjs',
+        'postcss.config.mjs',
         'tailwind.config.js',
       ],
       deps: [],
@@ -80,7 +80,7 @@ export const projectSpecification: ProjectSpecification = {
         'pages/_app.tsx',
         'pages/api/hello.ts',
         'pages/index.tsx',
-        'postcss.config.cjs',
+        'postcss.config.mjs',
         'tailwind.config.ts',
         'tsconfig.json',
       ],
@@ -125,7 +125,7 @@ export const projectSpecification: ProjectSpecification = {
         'app/layout.js',
         'app/page.js',
         'jsconfig.json',
-        'postcss.config.cjs',
+        'postcss.config.mjs',
         'tailwind.config.js',
       ],
     },
@@ -143,7 +143,7 @@ export const projectSpecification: ProjectSpecification = {
         'app/layout.tsx',
         'app/page.tsx',
         'next-env.d.ts',
-        'postcss.config.cjs',
+        'postcss.config.mjs',
         'tailwind.config.ts',
         'tsconfig.json',
       ],

test/integration/create-next-app/lib/utils.ts

@@ -135,7 +135,7 @@ export const shouldBeTemplateProject = ({
     files: getProjectSetting({ template, mode, setting: 'files', srcDir }),
   })
 
-  // Tailwind templates share the same files (tailwind.config.js, postcss.config.cjs)
+  // Tailwind templates share the same files (tailwind.config.js, postcss.config.mjs)
   if (template !== 'app-tw' && template !== 'default-tw') {
     projectFilesShouldNotExist({
       cwd,