Add Dictionary and Indexing Support Fix #29

[lwgray]

Description

This PR adds support for dictionaries and indexing operations to ThinkPy, enhancing its ability to teach data structure concepts and bringing it closer to Python's feature set.

Changes Made

1. Added dictionary literal syntax and AST nodes
2. Implemented indexing for lists and dictionaries
3. Added support for nested data structures
4. Enhanced error handling for invalid operations
5. Removed legacy tuple-based AST approach
6. Updated documentation

Example Usage

objective "Demonstrate new features"

task "Dictionary Operations" {
    step "Create and access dictionary" {
        user = {
            "name": "Alice",
            "scores": [85, 92, 78]
        }
        print(user["name"])           # Access dict value
        print(user["scores"][0])      # Nested access
    }
}

Testing Done

• Integration tests for nested structures
• Error handling tests for invalid operations
• Manual testing with complex data structures

Need these Documentation Updates

• Add dictionary and indexing sections to language reference
• Update quick start guide with new examples
• Add error handling documentation

Breaking Changes

None. All existing ThinkPy code will continue to work as before.

Related Issues

Closes #29

Checklist

• [x] Code follows ThinkPy style guide
• [ ] Tests added for new features
• [ ] Documentation updated
• [x] No breaking changes introduced
• [x] Error handling properly implemented