Implement 6 new Excel functions for Google Sheets

[Jython1415]

Summary

Implements 6 high-value Excel functions missing from Google Sheets, addressing issues #60, #54, #57, #63, #58, and #55.

Functions Implemented

Tier 1 - High-Value Quick Wins:

• ✅ PERCENTOF (Implement PERCENTOF function #60) - Calculate percentage contribution (subset sum / total sum)
• ✅ TEXTBEFORE (Implement TEXTBEFORE function #54) - Extract text before delimiter (mirrors TEXTAFTER)
• ✅ EXPAND (Implement EXPAND function #57) - Expand/pad arrays to specified dimensions
• ✅ SINGLE (Implement SINGLE function #63) - Extract single value from array intersection

Tier 2 - Strategic Additions:

• ✅ TRIMRANGE (Implement TRIMRANGE function #58) - Remove empty rows/columns from range edges
• ✅ TEXTSPLIT (Implement TEXTSPLIT function #55) - Split text into array (1D version with column delimiter)

Issues Closed

Also closed 3 issues as "not planned" with explanations:

• ❌ SORTBY (Implement SORTBY function #62) - Native SORT already provides this functionality
• ❌ VALUETOTEXT (Implement VALUETOTEXT function #64) - High complexity, low priority for current roadmap
• ❌ ARRAYTOTEXT (Implement ARRAYTOTEXT function #65) - Depends on Implement VALUETOTEXT function #64, low priority

Testing

• ✅ All 171 tests passing
• ✅ All 39 formulas pass lint checks
• ✅ README auto-generated and validated
• ✅ No circular dependencies

Implementation Details

All functions follow project standards:

• LET-based formula structure
• Fail-fast error handling
• No optional parameters (Google Sheets limitation)
• Non-empty parameter examples (linter enforced)
• Comprehensive pyparsing validation

Commits

1. Add PERCENTOF function to calculate percentage contribution
2. Add EXPAND function for array padding and dimension expansion
3. Add SINGLE function for extracting values from array intersections
4. Add TEXTSPLIT formula (issue Implement TEXTSPLIT function #55)
5. Add TRIMRANGE function to remove empty rows/columns from range edges
6. Add TEXTBEFORE function to extract text before delimiter

Test Plan

• Run linter: uv run python scripts/lint_formulas.py
• Generate README: uv run python scripts/generate_readme.py
• Run test suite: uv run pytest tests/ -v
• Verify all 171 tests pass
• Verify all 39 formulas pass lint
• Verify README updated correctly