function stripFrontMatter(mdFileString) {
  return mdFileString.replace(/^\s*---(.|\n)*?---/, '');
}
Frontmatter is problematic when traversing md files with
unist-util-select: https://github.com/syntax-tree/unist-util-select/blob/main/index.js.
So we remove it before parsing
@param {string} mdFileString
@returns {string}