How can I use Python to read data from a JSON file, transform the data by adding a new field based on existing fields, and write the transformed data back to a new JSON file?

Details:

I have a JSON file named "data.json" with an array of objects. Each object contains fields "name", "birth$\_$year", and "current$\_$year".

I want to add a new field "age" to each object, calculated as the difference between "current$\_$year" and "birth$\_$year".

I want to write the transformed data to a new JSON file named "transformed$\_$data.json".

Can you provide a Python script to accomplish this?