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__/format.test.ts

@@ -0,0 +1,52 @@
+/**
+ * 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 {getFormat} from '../format';
+
+describe('getFormat', () => {
+  it('uses frontMatter format over anything else', () => {
+    expect(getFormat({frontMatterFormat: 'md', filePath: 'xyz.md'})).toBe('md');
+    expect(getFormat({frontMatterFormat: 'md', filePath: 'xyz.mdx'})).toBe(
+      'md',
+    );
+    expect(getFormat({frontMatterFormat: 'mdx', filePath: 'xyz.md'})).toBe(
+      'mdx',
+    );
+    expect(getFormat({frontMatterFormat: 'mdx', filePath: 'xyz.mdx'})).toBe(
+      'mdx',
+    );
+  });
+
+  it('detects appropriate format from file extension', () => {
+    expect(getFormat({frontMatterFormat: 'detect', filePath: 'xyz.md'})).toBe(
+      'md',
+    );
+    expect(
+      getFormat({frontMatterFormat: 'detect', filePath: 'xyz.markdown'}),
+    ).toBe('md');
+
+    expect(
+      getFormat({frontMatterFormat: 'detect', filePath: 'folder/xyz.md'}),
+    ).toBe('md');
+    expect(
+      getFormat({frontMatterFormat: 'detect', filePath: 'folder/xyz.markdown'}),
+    ).toBe('md');
+    expect(getFormat({frontMatterFormat: 'detect', filePath: 'xyz.mdx'})).toBe(
+      'mdx',
+    );
+    expect(
+      getFormat({frontMatterFormat: 'detect', filePath: 'folder/xyz.mdx'}),
+    ).toBe('mdx');
+
+    expect(
+      getFormat({frontMatterFormat: 'detect', filePath: 'xyz.unknown'}),
+    ).toBe('mdx');
+    expect(
+      getFormat({frontMatterFormat: 'detect', filePath: 'folder/xyz.unknown'}),
+    ).toBe('mdx');
+  });
+});