Add parentheses/bracket/brace balance checks to linter

[Jython1415]

Summary

Add linter rules to validate balanced parentheses (), square brackets [], and curly braces {} in formulas.

Motivation

Commit c222245 introduced a syntax error (missing closing parenthesis) that passed linting but caused "Formula parse error" in Google Sheets. A balance check would have caught this immediately.

Requirements

1. Check balance of all three bracket types: (), [], {}
2. Handle string literals correctly - ignore brackets inside quoted strings
3. Provide clear error messages with position information
4. Run as part of existing linter workflow

Implementation Notes

• Add new BalancedBracketsRule class to scripts/lint_formulas.py
• Parse formula to identify string boundaries before checking balance
• Consider using simple state machine or regex for string detection

Reference

• Fixed in: Fix DENSIFY formula syntax error - missing closing parentheses #85
• Introduced in: c222245

[Jython1415]

This issue has been addressed by PR #120 (which implemented issue #108).

The ValidFormulaSyntaxRule added to the linter validates formulas using the pyparsing grammar, which inherently checks for balanced brackets, parentheses, and curly braces. The parser automatically detects unbalanced delimiters and reports syntax errors with position information.

Verification:

✅ Unbalanced parentheses: Detected
✅ Unbalanced curly braces: Detected
✅ Handles strings correctly: Yes (pyparsing grammar properly handles quoted strings)
✅ Provides position info: Yes (ParseException includes line/column data)

All requirements from this issue are now satisfied through the pyparsing-based validation.

Closing as resolved.