Add Comprehensive Operator Support and Improve Expression Parsing

[vagetman]

Changes Made

🔧 New Operators Added

• Equality operators: == (equals), != (not equals)
• Comparison operators: < (less than), <= (less than or equal), > (greater than), >= (greater than or equal)
• Logical operators: && (logical AND), || (logical OR)
• Unary operator: ! (negation)

🏷️ XML Attribute Parsing

• ESI include tag support: Full parsing of <esi:include src="..." alt="..." /> tags
• Attribute extraction: Proper handling of XML attributes with quoted values

🎯 Expression Features

• Type-aware comparison: Numeric comparison for integers, string comparison for text values
• Bareword support in variable subscripts: Allow unquoted identifiers in variable lookups (e.g., $(QUERY_STRING{param}))
• Contextual bareword parsing: Barewords only treated as strings in variable subscript position
• Parenthesized expressions: Support for (expression) syntax for grouping and precedence
• Enhanced error handling: Improved error messages and proper error propagation

🏗️ Architecture Changes

• Unified operator handling: Refactored to use Token::Operation(Operator) for all operators
• Improved lexer: Added support for multi-character operators
• Better parser structure: Enhanced maintainability and extensibility
• Type-aware evaluation: Different evaluation logic for numeric vs string comparisons

📚 Documentation

• Fixed doctests: Updated all code examples in documentation to compile and pass
• Improved examples: Added working examples for new operator functionality
• Better API docs: Enhanced documentation with accurate usage patterns

✅ Testing

• Added comprehensive tests for all new operators and operator combinations
• Added tests for bareword variable subscripts
• Added tests for ESI include tag parsing
• Added tests for complex expressions with negation and parentheses
• Added tests for type coercion and mixed expression types
• All existing tests continue to pass

Examples

ESI Include Tags

<!-- Include with expression in src -->
<esi:include src="/api/user/$(QUERY_STRING{user_id})" />

Standard HTML Tags with Attributes

<a href="/page/$(QUERY_STRING{id})">Dynamic Link</a>
<img src="/images/$(HTTP_HEADER{theme})/logo.png" alt="Logo" />

New Equality Operations

<esi:choose>
    <esi:when test="$(QUERY_STRING{param}) == 'value'">
        Parameter matches!
    </esi:when>
    <esi:when test="$(HTTP_HOST) != 'localhost'">
        Not running locally
    </esi:when>
</esi:choose>

Bareword Variable Subscripts

<esi:vars>
    $(QUERY_STRING{param})          <!-- New: bareword subscript -->
    $(QUERY_STRING{'param'})        <!-- Still supported: quoted -->
</esi:vars>

Complex Expressions with Negation

<esi:when test="!($(QUERY_STRING{debug}) == 'true')">
    Debug mode is off
</esi:when>

Breaking Changes

None - all changes are additive and maintain backward compatibility.

Performance Impact

Minimal - changes are primarily to parsing logic with no impact on runtime evaluation performance.

Ready for review 🚀

[kailan]

This looks fantastic, thank you so much for your work here!

I made a few minor suggestions, most of which are just clippy lints. Apart from that it looks good to go 👍

[kailan]

ship it!