Function builtin_from_json 

fn builtin_from_json(
    args: &[RValue],
    named: &[(String, RValue)],
) -> Result<RValue, RError>

Parse a JSON string into an R value.

Conversion rules:

• JSON object -> named list
• JSON array of objects (same keys) -> data.frame
• JSON array of scalars -> vector
• JSON null -> NULL
• JSON true/false -> logical
• JSON number -> double (or integer if representable)
• JSON string -> character

@param txt character scalar: JSON string to parse @return R value corresponding to the JSON structure