Gets the string scalar value when Kind is String or DateTime.
For String, this value is only populated when the parser was configured to decode scalars eagerly;
otherwise use GetString().
When EagerStringValues is enabled, this value may also be populated for simple basic strings
(single-line, no escape sequences) even when DecodeScalars is false.
For DateTime, this value contains the raw literal text so that consumers can parse it without
requiring DecodeScalars.
public string? StringValue { get; }