Improve LexicalAnalyzer number parsing

[mary-georgiou-sonarsource]

Currently, the LexicalAnalyzer class is parsing JSON integral numbers as double.
Considering that numbers in JSON can be an infinite series of digits (see specification) we should refactor the ReadNumberValue method to use long and BigInteger for integral, and double for decimals.
See draft of refactoring here.

This change should be thoroughly tested.