Feat: add the fmt_index() method

[rich-iannone]

This PR introduces a new "index" formatting feature that allows users to format numeric values as index characters (e.g., 1 → "A", 27 → "AA") in both the GT and vals APIs (as fmt_index()). The implementation includes support for both "repeat" and "excel" style algorithms, case selection, and custom patterns.

[codecov]

Codecov Report

❌ Patch coverage is 93.75000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.34%. Comparing base (284d28a) to head (6289e40).

Files with missing lines	Patch %	Lines
great_tables/_formats.py	92.68%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #888      +/-   ##
==========================================
- Coverage   98.36%   98.34%   -0.03%     
==========================================
  Files          52       52              
  Lines        8152     8200      +48     
==========================================
+ Hits         8019     8064      +45     
- Misses        133      136       +3     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.