Add General CSS Tokenizer and Parser in cssom/parsers

[Copilot]

This PR implements a comprehensive CSS tokenizer and parser to support advanced CSS features and improve maintainability throughout the JSAR Runtime engine.

Implementation Overview

The implementation provides two main components:

CSS Tokenizer (css_tokenizer.hpp/cpp)

A complete CSS tokenizer that handles all token types defined in the CSS Syntax Module Level 3:

• Basic tokens: identifiers, functions, strings, numbers, dimensions, percentages
• CSS-specific tokens: at-keywords (@media, @keyframes), hash tokens (#id, #color)
• Selector tokens: attribute selector operators (^=, $=, *=, |=, ~=)
• Structural tokens: brackets, braces, parentheses, semicolons, colons, commas
• Special tokens: CDO/CDC (<!-- -->), comments (filtered during tokenization)

CSS Parser (css_parser.hpp/cpp)

A general-purpose parser that can handle:

Complete Stylesheet Parsing:

• StyleRule - Standard style rules with complex selectors
• KeyframeRule - @keyframes animations
• FontFaceRule - @font-face declarations
• MediaRule - @media queries with nested rules
• ImportRule - @import statements

Single Style Declaration Parsing:

• Extracts property/value pairs from declaration strings
• Supports complex function values (linear-gradient(), rgba(), transform())
• Preserves important declarations (!important)
• Perfect for inline styles and style attributes

Example Usage

#include <client/cssom/parsers/css_tokenizer.hpp>
#include <client/cssom/parsers/css_parser.hpp>

// Parse complete stylesheet
CSSParser parser(R"(
  @media screen and (max-width: 768px) {
    .container { width: 100%; }
  }
  body { 
    background: linear-gradient(to right, red, blue);
    transform: rotate(45deg) scale(1.2);
  }
)");

auto rules = parser.parseStylesheet();
// Returns vector of parsed CSS rules

// Parse style declaration
auto result = parser.parseStyleDeclaration("color: red; font-size: 16px;");
// Returns: {properties: {"color": "red", "font-size": "16px"}, valid: true}

Key Features

• CSS Syntax Module Level 3 compliance for robust tokenization
• Complex selector support including combinators, pseudo-classes, and attribute selectors
• Function value preservation for gradients, transforms, and other CSS functions
• Error recovery with graceful handling of malformed CSS
• Memory-safe C++ implementation using RAII and smart pointers
• Comprehensive test coverage with both unit and integration tests

Architecture

The implementation follows existing codebase patterns and integrates seamlessly with the current CSS processing infrastructure. It extends the existing CSSValueTokenizer approach to handle full CSS syntax while maintaining consistency with parsers like CSSFilterParser and CSSTransformParser.

This provides a solid foundation for implementing more advanced CSS features throughout the engine, including better stylesheet processing, enhanced style attribute handling, and support for modern CSS syntax.

Fixes #303.

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