Preferve references in the frontmatter data

This means that objects generated from YAML anchors now point to the
same reference. The same goes for custom frontmatter parsers. Custom
parsers may now also return objects that contain recursive references.

fixtures/anchor/expected.jsx

@@ -0,0 +1,24 @@
+/*@jsxRuntime automatic*/
+/*@jsxImportSource react*/
+export const frontmatter = (() => {
+  const $0 = {
+    title: 'Hello frontmatter'
+  }
+  return {
+    original: $0,
+    reference: $0
+  }
+})()
+function _createMdxContent(props) {
+  return <></>
+}
+export default function MDXContent(props = {}) {
+  const { wrapper: MDXLayout } = props.components || {}
+  return MDXLayout ? (
+    <MDXLayout {...props}>
+      <_createMdxContent {...props} />
+    </MDXLayout>
+  ) : (
+    _createMdxContent(props)
+  )
+}

fixtures/anchor/input.md

@@ -0,0 +1,5 @@
+---
+original: &anchor
+  title: Hello frontmatter
+reference: *anchor
+---

src/remark-mdx-frontmatter.ts

@@ -75,7 +75,7 @@ const remarkMdxFrontmatter: Plugin<[RemarkMdxFrontmatterOptions?], Root> = ({
                   {
                     type: 'VariableDeclarator',
                     id: { type: 'Identifier', name },
-                    init: valueToEstree(data)
+                    init: valueToEstree(data, { preserveReferences: true })
                   }
                 ]
               }