Unify GT.cols_label() and GT.cols_width() to accept both cases and **kwargs

[jrycw]

This PR unifies GT.cols_label() and GT.cols_width() to accept both cases and **kwargs.

Currently, these two functions have different parameter structures, which can be confusing and often requires consulting the documentation. I hope this change simplifies their usage :)

[codecov]

Codecov Report

Attention: Patch coverage is 93.75000% with 1 line in your changes missing coverage. Please review.

Project coverage is 86.75%. Comparing base (fedac08) to head (95708a5).

Files with missing lines	Patch %	Lines
great_tables/_spanners.py	90.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #452   +/-   ##
=======================================
  Coverage   86.75%   86.75%           
=======================================
  Files          42       42           
  Lines        4702     4711    +9     
=======================================
+ Hits         4079     4087    +8     
- Misses        623      624    +1     

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[rich-iannone]

LGTM

[machow]

Thanks, this is great! I wonder if in the future, there might be some good uses of polars expressions inside cases, e.g.

cols_width({cs.starts_with("abc_"): "50px"})

Although maybe there's a more polars way to go from selector -> literal? 🤔