Feat: DataFrame functions subset_frame and to_column_dict

[machow]

This PR adds two new DataFrame generic functions:

• subset_frame: return a new DataFrame that is a subset of rows= and cols=.
• to_column_dict: convert dataframe to a dictionary, where each column is an entry.

These functions were used in prototyping rendering GT as an interactive table in https://github.com/machow/reactable-py/blob/main/reactable/render_gt.py, but I suspect they'll be useful here.

(Alternatively, we could implement them in reactable-py, which also has a _tbl_data.py file?!)

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 76.92308% with 6 lines in your changes missing coverage. Please review.

Project coverage is 86.41%. Comparing base (f403d46) to head (25072af). Report is 4 commits behind head on main.

Files	Patch %	Lines
great_tables/_tbl_data.py	81.81%	4 Missing :warning:
great_tables/_gt_data.py	50.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #415 +/- ## ========================================== + Coverage 86.40% 86.41% +0.01% ========================================== Files 42 42 Lines 4700 4720 +20 ========================================== + Hits 4061 4079 +18 - Misses 639 641 +2 ```

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[machow]

Going to close, since I moved the logic into reactable-py