Dropdown > Direction for rendering overflowing, non-wrapping text within items

In some cases, we might need to display long non-wrapping texts within a dropdown's items. For example, the notification settings display the user's emails, which might overflow the dropdown menu:

Implementation plan

• GlTruncate: ensure the rendered element has an accessible aria-label attribute.
• GlCollapsibleListbox
  • Make dropdown's width more flexible:
    • min-width: 15.5rem
    • max-width: 28.5rem
  • Leverage GlTruncate to truncate long items' names. This should be probably be the consumer's responsibility, so this can be a recommendation in the docs.
• Ensure the notifications settings render properly as per gitlab#373843 (comment 1227467886).