chore: update dependencies and improve markmap integration

- Updated markmap dependencies to versions 0.18.9 for markmap-common and 0.18.12 for markmap-lib and markmap-view.
- Refactored import statements in extra.js to use the updated markmap packages.
- Added a new helper function to sanitize markmap nodes against XSS vulnerabilities.

This update enhances the security and functionality of the markmap integration in the project.

Author: Yukaii