add wandb scalar writer

[AllanChain]

The W&B writer logs real-valued scalar statistics to Weights & Biases. It is not enabled by default, and the users should install wandb manually.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 90.38462% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.84%. Comparing base (525d61b) to head (1f9868c).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/jaqmc/writer/wandb.py	90.38%	3 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #42      +/-   ##
==========================================
+ Coverage   86.81%   86.84%   +0.03%     
==========================================
  Files         131      132       +1     
  Lines        6081     6133      +52     
  Branches      578      583       +5     
==========================================
+ Hits         5279     5326      +47     
- Misses        564      567       +3     
- Partials      238      240       +2     

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