chore: Added Sum of Digits Implementation (#684)

* Added the main logic, need to work on Tests

* Added tests for SOD

* Fix typo and add Wikipedia link in comments

* Fix mistake in SumOfDigitsUsingStrings

I intended to initially write a different implementation but I wrote something else 🤦‍♂️

* Converted Spacing from Tabs to Spaces

* Oops, forgot about the test file

* Fixed semicolon problems...

* Oops, I missed a few semicolons

* Linting is hell TwT

Co-authored-by: SpiderMath <{ID}+{username}@users.noreply.github.com>

Maths/test/SumOfDigits.test.js

@@ -0,0 +1,16 @@
+import { sumOfDigitsUsingLoop, sumOfDigitsUsingRecursion, sumOfDigitsUsingString } from '../SumOfDigits'
+
+test('Testing on sumOfDigitsUsingLoop', () => {
+  const sum = sumOfDigitsUsingLoop(123)
+  expect(sum).toBe(6)
+})
+
+test('Testing on sumOfDigitsUsingRecursion', () => {
+  const sum = sumOfDigitsUsingRecursion(123)
+  expect(sum).toBe(6)
+})
+
+test('Testing on sumOfDigitsUsingString', () => {
+  const sum = sumOfDigitsUsingString(123)
+  expect(sum).toBe(6)
+})