Refactor Learn transform in terms of new LabeledTable

[juliohm]

Necessary breaking change to generalize higher-level constructs in downstream packages.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 63.07692% with 24 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.85%. Comparing base (5a7e7ab) to head (534a93f).

Files with missing lines	Patch %	Lines
src/labeledtable.jl	41.02%	23 Missing ⚠️
src/learn.jl	96.15%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #16       +/-   ##
===========================================
- Coverage   95.61%   79.85%   -15.77%     
===========================================
  Files           7        7               
  Lines         114      134       +20     
===========================================
- Hits          109      107        -2     
- Misses          5       27       +22     

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