feat: add support for math blocks using $$

[OEvgeny]

Changelog Entry

Added

• Added support for math blocks using $$ delimiter alongside existing \[...\] and \(...\) notations, in PR #5381, by @OEvgeny

Description

This PR extends math block support by adding the $$ delimiter syntax. The implementation maintains consistency with existing math block handlers while introducing a new syntax option.

Design

The tokenizer implementation uses a state machine approach:

• Tracks consecutive $ characters via counter to distinguish between potential math delimiters
• Only commits to math mode after exactly two $ characters ($$) are detected
• Uses unified state handling for all delimiters ($$, \[...\], \(...\)) with single expected close delimiter variable
• Rejects mismatched delimiters (e.g., $$$) to prevent incorrect parsing

Specific Changes

• Updated math tokenizer to handle $$ delimiters for display math mode
• Modified HTML renderer to process new delimiter syntax
• Added test coverage for mixed math blocks usage with various mathematical expressions
• Reorganized code structure for better maintainability:
  • Split math extension into separate files
  • Renamed htmlRenderer.ts to mathHtml.ts for clarity

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

Review Checklist

This section is for contributors to review your work.

• 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)