feat: upgrade to MDX v2 (#8288)

Co-authored-by: Armano <armano2@users.noreply.github.com>

packages/docusaurus-mdx-loader/src/frontMatter.ts

@@ -0,0 +1,31 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+import {
+  JoiFrontMatter as Joi,
+  validateFrontMatter,
+} from '@docusaurus/utils-validation';
+
+export type MDXFrontMatter = {
+  format: 'md' | 'mdx' | 'detect';
+};
+
+export const DefaultMDXFrontMatter: MDXFrontMatter = {
+  format: 'detect',
+};
+
+const MDXFrontMatterSchema = Joi.object<MDXFrontMatter>({
+  format: Joi.string()
+    .equal('md', 'mdx', 'detect')
+    .default(DefaultMDXFrontMatter.format),
+}).default(DefaultMDXFrontMatter);
+
+export function validateMDXFrontMatter(frontMatter: unknown): MDXFrontMatter {
+  return validateFrontMatter(frontMatter, MDXFrontMatterSchema, {
+    allowUnknown: false,
+  });
+}