Add nCr function to math directory

This PR adds a function to calculate the number of combinations (n choose r) in Kotlin.
It handles input validation and efficiently computes the result using an iterative approach.
Example tests are included in the main() function.

Author: PauLopNun

src/main/kotlin/math/nCr.kt

@@ -0,0 +1,17 @@
+/**
+ * Computes the number of combinations (n choose r)
+ */
+fun nCr(n: Int, r: Int): Long {
+    require(n >= 0 && r >= 0 && r <= n) { "Invalid n or r" }
+    var result = 1L
+    for (i in 1..r) {
+        result = result * (n - i + 1) / i
+    }
+    return result
+}
+
+fun main() {
+    println("C(5,2) = ${nCr(5,2)}")  // 10
+    println("C(10,3) = ${nCr(10,3)}") // 120
+    println("C(6,0) = ${nCr(6,0)}")  // 1
+}