chore: install mdx2vast on GitLab

Author: jdkato

.gitlab-ci.yml

@@ -16,6 +16,7 @@ test:
 
     - apt install -y python3-sphinx
     - gem install asciidoctor bundler
+    - npm install -g mdx2vast
 
     - wget https://github.com/dita-ot/dita-ot/releases/download/3.6/dita-ot-3.6.zip
     - unzip dita-ot-3.6.zip > /dev/null 2>&1