mdx: add dependency on acorn

Yarn results in trouble for peer dependencies.
This solves that.

Related-to: mdx-js/recma#3.

Closes GH-2634.

Author: wooorm

package-lock.json

@@ -47,6 +47,7 @@
     "@types/estree-jsx": "^1.0.0",
     "@types/hast": "^3.0.0",
     "@types/mdx": "^2.0.0",
+    "acorn": "^8.0.0",
     "collapse-white-space": "^2.0.0",
     "devlop": "^1.0.0",
     "estree-util-is-identifier-name": "^3.0.0",