feat: implement BETWEEN, IN, LIKE, IS NULL operators in WHERE clause (#117)

* feat: implement BETWEEN, IN, LIKE, IS NULL operators in WHERE clause

- Add BETWEEN/NOT BETWEEN expression parsing with AND keyword handling
- Add IN/NOT IN expression parsing with parenthesized value lists
- Add LIKE/NOT LIKE/ILIKE pattern matching operators
- Add IS NULL/IS NOT NULL postfix operators
- Handle NOT prefix properly for negated operators
- Update parser_coverage_test.go expectations (5 tests now pass)
- Add comprehensive operators_test.go with 14 test cases

SQL-99 compliance improvements for comparison predicates.

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Co-Authored-By: Claude <noreply@anthropic.com>

* fix: correct NOT token handling with look-ahead parsing

Address PR review comment about NOT token handling breaking valid SQL.

Changes:
- Add peekToken() helper for look-ahead without advancing position
- Fix parseComparisonExpression() to only consume NOT when followed by
  BETWEEN, LIKE, ILIKE, or IN operators
- Add NOT case in parsePrimaryExpression() for standalone boolean negation
  (e.g., WHERE NOT active)

This properly handles all NOT use cases:
- NOT LIKE: WHERE name NOT LIKE '%Admin%'
- NOT BETWEEN: WHERE price NOT BETWEEN 10 AND 100
- NOT IN: WHERE status NOT IN ('cancelled', 'deleted')
- Boolean negation: WHERE NOT active AND name LIKE '%'

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

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Co-authored-by: Ajit Pratap Singh <ajitpratapsingh@Ajits-Mac-mini.local>
Co-authored-by: Claude <noreply@anthropic.com>

Author: 3 people