Add support for contentClass prop

[stan-sack]

Background

• I've recently run into some issues with the content component where I am unable to set overflow: visible on the div and an absolute div inside is getting cut off. I need to be able to pass a className to the content container, it doesnt work to just set the style on the main Panel div.

Changes

• Add support for contentClass prop

Tests

• Existing unit tests pass
• Verified that the feature works in my own project by building and importing the newly built Panel

[vercel]

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🔍 Inspect: https://vercel.com/react-component/collapse/nasta7l59
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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.43%. Comparing base (b5d62a6) to head (817041e).
⚠️ Report is 103 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #166   +/-   ##
=======================================
  Coverage   97.43%   97.43%           
=======================================
  Files           5        5           
  Lines         117      117           
  Branches       35       35           
=======================================
  Hits          114      114           
  Misses          3        3           

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