Test: add more cases for three digits convert to one number string with two words
TPP: constant -> constants, plus some small refactoring
Test: 1000
TPP: constant -> constants
Add acceptance test 1234
Test: add more cases, 2000 - 9000
TPP: constant -> constants
Test: 10,000
TPP: constant -> constants
But, I see some big duplication of data (NUMBER-STRINGS). I will do some refactoring before moving forward.