Add .writer.json

[khaeru]

Will close #35.

PR checklist

• Checks all ✅
• Update documentation
• Update doc/whatsnew.rst

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.27%. Comparing base (9a02ccb) to head (d34482c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #253      +/-   ##
==========================================
- Coverage   99.03%   98.27%   -0.77%     
==========================================
  Files         115      121       +6     
  Lines        9360     9438      +78     
==========================================
+ Hits         9270     9275       +5     
- Misses         90      163      +73     

Files with missing lines	Coverage Δ
sdmx/__init__.py	100.00% <100.00%> (ø)
sdmx/convert/common.py	100.00% <100.00%> (ø)
sdmx/format/json.py	100.00% <100.00%> (ø)
sdmx/tests/writer/conftest.py	100.00% <100.00%> (ø)
sdmx/tests/writer/test_json.py	100.00% <100.00%> (ø)
sdmx/writer/json/__init__.py	100.00% <100.00%> (ø)
sdmx/writer/json/base.py	100.00% <100.00%> (ø)
sdmx/writer/json/v1.py	100.00% <100.00%> (ø)
sdmx/writer/json/v2.py	100.00% <100.00%> (ø)

... and 21 files with indirect coverage changes

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