Add default handler for non-JSON-serializable objects

[brandonvin]

With this PR, the :json formatter now handles objects that aren't JSON serializable by calling clojure.core/str on them and writing the resulting string. Previously, these values would cause the event to be dropped with a warning similar to:

[dialog output error] Failed to write to output stdout: Don't know how to write JSON of class <class>

I updated the clojure.data.json dependency to the latest version to pick up the new :default-write-fn option used to implement this.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.68%. Comparing base (06efe04) to head (705a1d1).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #41      +/-   ##
==========================================
+ Coverage   94.66%   94.68%   +0.02%     
==========================================
  Files           8        8              
  Lines         618      621       +3     
  Branches       13       13              
==========================================
+ Hits          585      588       +3     
  Misses         20       20              
  Partials       13       13              

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