function isInsideExisting (newItem, existing) {
  const existingIntervalSize = existing.to - existing.from
  const newItemIntervalSize = newItem.to - newItem.from
  return newItem.from >= existing.from && newItem.to <= existing.to && (newItemIntervalSize < existingIntervalSize)
}
Examples:
existing: 10-20 | 10-20 | 10-20
new_item: 12-15 | 20-20 | 15-20
@param {MinimalSelectionItem} newItem
@param {MinimalSelectionItem} existing
@returns {boolean} returns true if the new item is completely inside the existing item