Implement CSS attribute selector support in CSSOM parser and matcher

[Copilot]

This PR implements comprehensive CSS attribute selector support in the CSSOM engine, enabling parsing and matching of selectors like link[rel="modulepreload"] as requested in the issue.

Changes Made

Parser Implementation

• Added ComponentType::kAttribute enum value for attribute selectors
• Added AttributeMatchType enum supporting all CSS attribute selector operators:
  • [attr] - attribute existence
  • [attr="value"] - exact match
  • [attr^="prefix"] - prefix match
  • [attr$="suffix"] - suffix match
  • [attr*="substring"] - substring match
  • [attr~="word"] - whitespace-separated word match
  • [attr|="prefix"] - dash-separated prefix match
• Extended Component class with attribute-specific fields and accessor methods
• Implemented parseAttributeSelector() method with proper handling of quoted/unquoted values and escape sequences

Matcher Implementation

• Added attribute matching logic in matchesSelectorComponentNonCombinator()
• Implemented all attribute match types according to CSS specification
• Added proper whitespace and dash-prefix matching algorithms

Test Coverage

• Comprehensive test suite covering all attribute selector variations
• Specific tests for the requested link[rel="modulepreload"] example
• Edge cases including spaces, quotes, compound selectors, and unquoted values
• String representation tests ensuring proper CSS serialization

Example Usage

The implementation now supports selectors like:

/* Resource hints - the main use case from the issue */
link[rel="modulepreload"] { /* styles */ }
link[rel="preload"] { /* styles */ }

/* Form styling */
input[type="text"] { /* styles */ }
input[disabled] { /* styles */ }

/* URL-based styling */
a[href^="https"] { /* styles */ }
img[src$=".jpg"] { /* styles */ }

/* Class-based matching */
div[class*="highlight"] { /* styles */ }
p[class~="button"] { /* styles */ }

/* Language attributes */
html[lang|="en"] { /* styles */ }

This enables advanced CSS features for resource hints, form styling, accessibility improvements, and progressive enhancement patterns.

Fixes #270.

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