JSON Format Support for GNDS Data in Fudge

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### Question
I have been working with GNDS data and found the support for converting between ENDF and XML formats quite useful. However, I was wondering if there is any native support in Fudge for converting GNDS data to and from JSON format? JSON is a widely-used and convenient format for many applications, especially when integrating with other tools and pipelines.

### Workaround
For now, I am using the following Python code to convert GNDS XML data to JSON format:

```python
import xml.etree.ElementTree as ET
import json

def xml_to_dict(elem):
d = {elem.tag: {} if elem.attrib else None}
children = list(elem)
if children:
dd = {}
for dc in map(xml_to_dict, children):
for k, v in dc.items():
if k in dd:
if not isinstance(dd[k], list):
dd[k] = [dd[k]]
dd[k].append(v)
else:
dd[k] = v
d = {elem.tag: dd}
if elem.attrib:
d[elem.tag].update(('@' + k, v) for k, v in elem.attrib.items())
if elem.text:
text = elem.text.strip()
if children or elem.attrib:
if text:
d[elem.tag]['#text'] = text
else:
d[elem.tag] = text
return d

def convert_gnds(xml_file, json_file):
tree = ET.parse(xml_file)
root = tree.getroot()
gnds_dict = xml_to_dict(root)
with open(json_file, 'w') as f:
json.dump(gnds_dict, f, indent=2)

convert_gnds('gnds_data.xml', 'gnds_data.json')
```

The issue names no Fudge files or tests; start by locating the existing ENDF/XML conversion entry points and compare their GNDS representations with the supplied Python xml_to_dict and convert_gnds workaround. Done means Fudge can convert GNDS data both to and from JSON with a defined mapping and verified round trips.

Written by the indexing model from the issue text.