Set non-JSON-schema-defined is<type> functions to reflect the internal data storage type

[TTF2050]

Closes #1361

Methods referencing schema-defined properties reflect the schema definitions. Methods referencing C++ data types reflect the underlying storage, as this is important for deciding how to extract a value without loss of precision. Using a combination of is<type> and isConvertibleTo methods should be sufficient for understanding whether a given value can be extracted as<type> with possible precision loss.