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.
include .js instead of .ts

I do make this mistake often.
Merge branch 'newApp-02-02' into newApp-02

Conflicts:
	MathPractice/MathPractice/default.htm
include .js instead of .ts

Because I make this change often
fix conflicted files
include both files correctly

Because then it works correctly
Merge branch 'newApp-02-03' into newApp-02
Move operand fields into the base class

Because these are common too