Extract JSConverter into a JSConverter.h file

[vmoroz]

The JSConverter is added for value conversion in JSValue.cpp. The JSValueReader and JSValueWriter also need to do similar conversions, but currently they use different algorithms. It would beneficial that all conversions are done uniformly between these classes. E.g. there is special logic how to convert NaN to a string.
The change will require to move the JSConverter implementation into its own header file JSConverter.h and change JSValue, JSValueReader, and JSValueWriter to use this new class.