feat: add syntax highlighting into markdown code block

[OEvgeny]

Changelog Entry

Added

• Introduced syntax highlighting for markdown code blocks, in PR #5389, by @OEvgeny

Description

This enhances code block rendering in markdown by leveraging custom elements architecture to provide better integration between React components and markdown output. This enables direct property binding and updates to
custom elements (code block, copy button).

Key improvements:

• Decouple code block rendering from HTML transform middleware, enabling direct property updates
• Provide foundation for custom syntax highlighters and theme customization
• Use copy button as independent component from code block custom element
• Introduce useUpdater hook to handle communication between React and custom elements

Design

The implementation follows Web Chat's composer pattern for consistency:

1. CodeBlock custom element handles the core rendering and updates
2. ShikiComposer manages highlighting configuration
3. useUpdater hook provides synchronization between React state and custom element updates

Moving highlights configuration to bundle package to align with Web Chat's architecture.

Specific Changes

• Introduced CodeBlock custom element with support for theme and language attributes
• Added ShikiComposer to bundle package for centralized highlight management
• Created useUpdater hook for managing React-to-custom-element state updates
• Updated HTML transform middleware to work with new code block architecture
• Updated snapshots and add more tests

• I have added tests and executed them locally
• I have updated CHANGELOG.md
• I have updated documentation

Review Checklist

• Accessibility reviewed (tab order, content readability, alt text, color contrast)
• Browser and platform compatibilities reviewed
• CSS styles reviewed (minimal rules, no z-index)
• Documents reviewed (docs, samples, live demo)
• Internationalization reviewed (strings, unit formatting)
• package.json and package-lock.json reviewed
• Security reviewed (no data URIs, check for nonce leak)
• Tests reviewed (coverage, legitimacy)