Create count_vowels.py (#11474)

* Create count_vowels.py

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strings/count_vowels.py

@@ -0,0 +1,34 @@
+def count_vowels(s: str) -> int:
+    """
+    Count the number of vowels in a given string.
+
+    :param s: Input string to count vowels in.
+    :return: Number of vowels in the input string.
+
+    Examples:
+    >>> count_vowels("hello world")
+    3
+    >>> count_vowels("HELLO WORLD")
+    3
+    >>> count_vowels("123 hello world")
+    3
+    >>> count_vowels("")
+    0
+    >>> count_vowels("a quick brown fox")
+    5
+    >>> count_vowels("the quick BROWN fox")
+    5
+    >>> count_vowels("PYTHON")
+    1
+    """
+    if not isinstance(s, str):
+        raise ValueError("Input must be a string")
+
+    vowels = "aeiouAEIOU"
+    return sum(1 for char in s if char in vowels)
+
+
+if __name__ == "__main__":
+    from doctest import testmod
+
+    testmod()