fix: allow html syntax in MDX v2 with format md (#8960)

* attempt to support html embeds in mdx with format md

* refactor mdx loader + support embedding html in commonmark thanks to rehype-raw

* extract processor code

* refactor processor code

* extract format + unit test

* try to refactor processor

* try to refactor processor

* adjust md page

* do not apply rehype-raw when format is mdx

* fix lint issue

packages/docusaurus-mdx-loader/src/__tests__/processor.test.ts

@@ -0,0 +1,33 @@
+/**
+ * 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 {createProcessor} from '../processor';
+// import type {Options} from '../loader';
+
+/*
+async function testProcess({
+  format,
+  options,
+}: {
+  format: 'md' | 'mdx';
+  options: Options;
+}) {
+  return async (content: string) => {
+    const processor = await createProcessor({format, options});
+    return processor.process(content);
+  };
+}
+ */
+
+describe('md processor', () => {
+  it('parses simple commonmark', async () => {
+    // TODO no tests for now, wait until ESM support
+    // Jest does not support well ESM modules
+    // It would require to vendor too much Unified modules as CJS
+    // See https://mdxjs.com/docs/troubleshooting-mdx/#esm
+  });
+});