message: Fix `collapsible-content-button` on Firefox
  It would discard the selected content instead of wrapping it
diff: @@ -45,13 +45,14 @@ function init(): void {
  function addContentToDetails(event: DelegateEvent<MouseEvent, HTMLButtonElement>): void {
  const field = event.delegateTarget.form!.querySelector('textarea')!;
  + const selection = field.value.slice(field.selectionStart, field.selectionEnd);
  // Don't indent <summary> because indentation will not be automatic on multi-line content
  const newContent = `
  <details>
  <summary>Details</summary>
  - ${getSelection()!.toString()}
  + ${selection}
  </details>
  `.replace(/(\n|\b)\t+/g, '$1').trim();
  