Write a function first_a_then_b(list_ab: list) -> list that given a list of strings, returns a new list with all the "a"s at the front, then all the "b"s. Any other characters (like "c","d"...) must be removed.
For example:
first_a_then_b(["a","b","a"])->["a","a","b"]first_a_then_b(["a","d"])->["a"]first_a_then_b(["a","c","a","b","a","a","ab"])->["a","a","a","a","b"]