Add prop to `gl-dropdown-item` to prevent dropdown from closing on clicking item

This would make the dropdown menu remain open after the user clicks on one of its items.

An enhancement in EE required this. This kind of behaviour is useful if you think the user might want to interact with items in a dropdown several times in a row; perhaps it's emulating a select[multiple], for example, or toggling reactive filters in a chart.

This probably only makes sense when the item is not a link, i.e., does not have an href, since that would imply a navigation.

Because the underlying b-dropdown-item-button always fires a clicked::link event on the $root, which closes the dropdown, and provides no way to override that behaviour, we may have to reimplement b-dropdown-item-button ourselves.

For example, the new prop could be simply:

noCloseDropdown: {
  type: Boolean,
  default: false,
  required: false,
}