Feat cell merge

[machow]

This PR adds our very rough sketch of how cell merging could be implemented

[codecov]

Codecov Report

❌ Patch coverage is 91.07143% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.43%. Comparing base (5196b53) to head (280fa3f).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
great_tables/_gt_data.py	91.66%	4 Missing ⚠️
great_tables/_tbl_data.py	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #735      +/-   ##
==========================================
- Coverage   91.43%   91.43%   -0.01%     
==========================================
  Files          47       47              
  Lines        5547     5602      +55     
==========================================
+ Hits         5072     5122      +50     
- Misses        475      480       +5     

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[rich-iannone]

Thanks for this PR, will push tests to this branch!