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 :man_facepalming:

* 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>