Add count_digits function with test cases

This PR adds a simple function to count the number of digits in an integer.
Includes basic test cases using assert. Follows the style used in other math algorithms.

Author: Lumowhisp

math/count_digits.cpp

@@ -0,0 +1,27 @@
+#include <iostream>
+#include <cassert>
+
+// returns how many digits the number has
+int count_digits(int n) {
+    if (n == 0) return 1;
+
+    int c = 0;
+    while (n > 0) {
+        c++;
+        n /= 10;
+    }
+    return c;
+}
+
+void test() {
+    assert(count_digits(12345) == 5);
+    assert(count_digits(9) == 1);
+    assert(count_digits(1000) == 4);
+    assert(count_digits(0) == 1);
+}
+
+int main() {
+    test();
+    std::cout << "Tests passed\n";
+    return 0;
+}