refactor(icon): Enhanced multi-theme support and improved performance

[rkaraivanov]

Description

Features:

• Add support for multiple themes coexisting on the same page
• Icon references now resolve based on per-component theme context
• Icons within different igc-theme-provider elements render correctly
• Implement batched notifications using microtask queue for performance
• Add automatic deduplication of icon change notifications

Performance Improvements:

• Refactor _iconLoaded to class method instead of arrow function property
  • Reduces memory footprint by sharing method across all icon instances
  • Still maintains proper lifecycle management with bound references
• Implement notification batching to prevent excessive listener calls
  • Uses queueMicrotask for zero-delay batching
  • Automatically deduplicates icon:collection pairs
  • Significantly improves performance when registering multiple icons

Architecture Changes:

• Remove global theme state from IconsRegistry singleton
• Pass theme as parameter to getIconRef() for stateless resolution
• Icon components now pass their local theme context to registry
• Each icon resolves references based on its own theme-provider context
• Preserve backward compatibility with optional theme parameter

Icon Resolution Priority (updated):

1. User-set references (external or internal)
2. Theme-specific aliases from ICON_REFERENCES
3. Default theme fallback
4. Return icon as-is if no reference found

Added API docuementation for all new and updated methods, properties, and behaviors. Added comprehensive unit and integration tests covering multi-theme scenarios, BFCache handling, and theme-provider interactions.

Fixes:

• Icons now work correctly in multi-theme applications
• No more theme conflicts when using multiple theme-providers
• Prevents unnecessary re-renders during bulk icon registration
• Eliminates potential race conditions in theme state

This refactor enables proper multi-theme support while maintaining backward compatibility and improving overall performance through batched notifications.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Documentation update
• Refactoring (code improvements without functional changes)

Checklist

• My code follows the project's coding standards
• I have tested my changes locally
• I have updated documentation if needed