Add a method in JsonParser to allow checking for "NaN" values

[cowtowncoder]

Currently some backends allow use and decoding of various NaN values (like negative/positive infinity, not-a-number from division by zero), but there is no way to check whether value might be such.
This can complicate handling for cases like:

FasterXML/jackson-databind#1315

because whereas double/Double may represent such values, BigDecimal can not.

So it would make sense to allow checking for such status, if that is feasible to do.