Turbopack: Add support for extension regex in @next/mdx (#78734)

Closes #78563 (replaces it)
Resolves #78348

This allows users the same flexibility they have with `@next/mdx` for webpack. It depends on #78733.

Test Plan: Included the integration test from #78563

Author: wbinnssmith

packages/next-mdx/index.js

@@ -49,11 +49,16 @@ module.exports =
     if (process.env.TURBOPACK) {
       nextConfig.turbopack = Object.assign({}, nextConfig?.turbopack, {
         rules: Object.assign({}, nextConfig?.turbopack?.rules, {
-          '*.mdx': {
+          '#next-mdx': {
             loaders: [loader],
             as: '*.tsx',
           },
         }),
+        conditions: {
+          '#next-mdx': {
+            path: extension,
+          },
+        },
         resolveAlias: Object.assign({}, nextConfig?.turbopack?.resolveAlias, {
           'next-mdx-import-source-file':
             '@vercel/turbopack-next/mdx-import-source',

test/e2e/app-dir/mdx/pages/pages/import/index.tsx

@@ -1,8 +1,9 @@
-import MDXContent from './mdx-file.mdx'
+import MDContent from './md-file.md'
+
 export default function Page() {
   return (
     <>
-      <MDXContent />
+      <MDContent />
     </>
   )
 }

test/e2e/app-dir/mdx/pages/pages/import/mdx-file.mdx renamed to test/e2e/app-dir/mdx/pages/pages/import/md-file.md

@@ -4,3 +4,9 @@ declare module '*.mdx' {
   let MDXComponent: (props) => JSX.Element
   export default MDXComponent
 }
+
+declare module '*.md' {
+  import type { JSX } from 'react'
+  let MDXComponent: (props) => JSX.Element
+  export default MDXComponent
+}