Comply with ESM. Convert live code example to Jest test.

Maths/test/PermutationAndCombination.test.js

@@ -0,0 +1,19 @@
+import { factorial, permutation, combination } from '../PermutationAndCombination'
+
+describe('Factorial', () => {
+  it('factorial(5)', () => {
+    expect(factorial(5)).toBe(120)
+  })
+})
+
+describe('Permutation', () => {
+  it('permutation(5, 2)', () => {
+    expect(permutation(5, 2)).toBe(20)
+  })
+})
+
+describe('Combination', () => {
+  it('combination(5, 2)', () => {
+    expect(combination(5, 2)).toBe(10)
+  })
+})