Log status messages to stderr if using JSON/YAML output

[adamchalmers]

The Zoo CLI lets you specify an output format, including human-friendly output formats (like plaintext or tables) and machine-readable ones (JSON, YAML).

Human-friendly formats should log status messages (like "processed 4 files" or "wrote image to foo.png") to stdout, because that's where humans expect them. But for machine-readable formats (like json), they should log statuses to stderr, to avoid breaking parsers if you do zoo | jq.

Closes #1757

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 66.18%. Comparing base (c8505dd) to head (5fba7b0).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1845      +/-   ##
==========================================
+ Coverage   66.11%   66.18%   +0.06%     
==========================================
  Files          43       43              
  Lines        8434     8451      +17     
==========================================
+ Hits         5576     5593      +17     
  Misses       2858     2858              

Flag	Coverage Δ
unittests	66.18% <100.00%> (+0.06%)	⬆️

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