function check(value) {
  // To do: investigate if we can change ` ```jsx ` -> ` ```js `?
  if (value === 'jsx' || value === 'mdx-invalid') return undefined
  return checkGithubLinguistFlag(value)
}
Check according to GitHub Linguist.

@param {string} value
  Language flag to check.
@returns {string | undefined}
  Whether the flag is valid (`undefined`),
  or a message to warn about (`string`).
@satisfies {CheckFlag}