Add If Missing

You get a shopping list items, a list of strings, and a word item. Add item at the end only when it isn't in the list yet. Return the list.

When item is already there, the list stays as it is. The list may start empty.

Example 1:

Input: items = ["milk", "eggs"], item = "bread"
Output: ["milk", "eggs", "bread"]
Explanation: The word "bread" is not in the list yet.
It goes at the end.

Example 2:

Input: items = ["milk", "eggs"], item = "milk"
Output: ["milk", "eggs"]
Explanation: The word "milk" is already there.
Nothing is added.

Example 3:

Input: items = [], item = "apples"
Output: ["apples"]
Explanation: The list is empty, so "apples" is new.
It becomes the only item.

Constraints:

• items has 0 to 9 words: it may be empty.
• item and every word in items are made of lowercase English letters.