fix: allow string arrays when serializing Altair settings

[nekosaur]

See #1204

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.26%. Comparing base (a84c553) to head (538ddd9).
⚠️ Report is 36 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #1205       +/-   ##
===========================================
- Coverage   90.88%   68.26%   -22.63%     
===========================================
  Files          50      123       +73     
  Lines        2425     4528     +2103     
  Branches      432      548      +116     
===========================================
+ Hits         2204     3091      +887     
- Misses        175     1339     +1164     
- Partials       46       98       +52     

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