Update JSON output to follow PS3.18 Annex F

[kabaluyot]

The JSON output formatting doesnt follow the output like in https://pydicom.github.io/pydicom/dev/tutorials/dicom_json.html, https://www.dicomstandard.org/, and https://support.dcmtk.org/docs/dcm2json.html.

The output kinda have custom tags like Group, Element, valueLength (please correct if this is not the intended output)

[suyashkumar]

Hi there! The JSON representation in this library was initially designed for debug output (it's pretty tied to the Go representations/structures), but I think it's very reasonable to make it interoperable since there's a JSON model for dicom in PS3.18 Annex F: https://dicom.nema.org/medical/dicom/current/output/chtml/part18/chapter_F.html

I'll need to dig into the specification more, but this may require some transformations to do since iirc the current json serialization is tied to the Go structures/representations.

Will mark this issue accordingly