fix(nextjs): add support next.config.mts for mdx (#2550)

.changeset/bright-singers-rescue.md

@@ -0,0 +1,5 @@
+---
+"fumadocs-mdx": patch
+---
+
+add support next.config.mts for mdx

packages/mdx/src/bin.ts

@@ -7,6 +7,7 @@ async function start() {
   const isNext =
     existsSync('next.config.js') ||
     existsSync('next.config.mjs') ||
+    existsSync('next.config.mts') ||
     existsSync('next.config.ts');
 
   if (isNext) {