Put the AdditionProblem in a module

Starting the refactoring journey
Put AdditionProblem in a new file

Starting to refactor into modules.
Introduce base class

Because there are similarities on math problems.
Create base class of MathProblem

Notice that I also move some code around in the constructor to
facilitate the next step
Move common code to the base class

Notice that base class fields are protected by default.
fix merge conflict
include .js instead of .ts

Because I make this change often
Move operand fields into the base class

Because these are common too