fix: use default submodules export in package.json (#8330)

Use `default` submodules export in `package.json` to ensure compatibility, as specified in https://nodejs.org/api/packages.html#conditional-exports

packages/next-auth/package.json

@@ -29,34 +29,34 @@
   "exports": {
     ".": {
       "types": "./index.d.ts",
-      "import": "./index.js"
+      "default": "./index.js"
     },
     "./adapters": {
       "types": "./adapters.d.ts"
     },
     "./jwt": {
       "types": "./jwt/index.d.ts",
-      "import": "./jwt/index.js"
+      "default": "./jwt/index.js"
     },
     "./react": {
       "types": "./react/index.d.ts",
-      "import": "./react/index.js"
+      "default": "./react/index.js"
     },
     "./next": {
       "types": "./next/index.d.ts",
-      "import": "./next/index.js"
+      "default": "./next/index.js"
     },
     "./middleware": {
       "types": "./middleware.d.ts",
-      "import": "./middleware.js"
+      "default": "./middleware.js"
     },
     "./client/_utils": {
       "types": "./client/_utils.d.ts",
-      "import": "./client/_utils.js"
+      "default": "./client/_utils.js"
     },
     "./providers/*": {
       "types": "./providers/*.d.ts",
-      "import": "./providers/*.js"
+      "default": "./providers/*.js"
     }
   },
   "scripts": {