function normalizeLinguistExtension(d) {
  // Extensions are case-insensitive (example: for `.OutJob`, `.outjob` also works).
  // They can also contain dots, dashes, plusses, etc.
  const normal = d.toLowerCase()
  assert.ok(/^\.[\w+.-]+$/.test(normal), normal)
  return normal
}
@param {string} d
  Extension to normalize.
@returns {string}
  Normalized extension.