Resume using upstream react-mosaic-component

[marlo-longley]

This PR would replace #4125 and copies the approach of @geourjoa in his forked PR (geourjoa#2). Thanks for sketching out that path forward.

I found one remaining issue in that fork — the type: 'split' field and updated children array syntax were needed to resolve the "Unknown mosaic node type" error in v7's n-ary tree format.

Ready for review/merge, but worth discussing whether we should wait til this is out of beta.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.9%. Comparing base (ecb376e) to head (36674ff).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #4279     +/-   ##
=======================================
- Coverage   92.9%   92.9%   -0.0%     
=======================================
  Files        335     335             
  Lines       5536    5534      -2     
  Branches    1958    1959      +1     
=======================================
- Hits        5141    5139      -2     
  Misses       348     348             
  Partials      47      47             

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