(next/mdx) set providerImportSource to react by default (#39954)

Hello, this PR simply configures the `next/mdx` package to use `{ providerImportSource: '@mdx-js/react' }` by default. This is now [required in version 2](https://mdxjs.com/packages/mdx/#optionsproviderimportsource) and causes a lot of confusion personally because `MDXProvider` seems to not work when all is needed is to set this option.

Co-authored-by: JJ Kasper <jj@jjsweb.site>

packages/next-mdx/index.js

@@ -11,7 +11,10 @@ module.exports =
             options.defaultLoaders.babel,
             {
               loader: require.resolve('@mdx-js/loader'),
-              options: pluginOptions.options,
+              options: {
+                providerImportSource: '@mdx-js/react',
+                ...pluginOptions.options,
+              },
             },
           ],
         })