-----Description----- 
This task requires writing a Lean 4 method that determines whether a given string contains all 5 English vowels: a, e, i, o, u.

The check is case-insensitive, meaning that both uppercase and lowercase vowels count.

-----Input-----
The input consists of a string:
s: A string of alphabetic characters (may include uppercase and lowercase)

-----Output-----
The output is true or false:
Returns true if the input string contains all 5 vowels (a, e, i, o, u), false otherwise.

