markdown-it v15 tracking

[brc-dd]

• fix upstream types
• replace mathjax plugin with something from mdit-plugins
• mdit-plugins support
• mdit-vue support
• markdown-it-async support
• bump and move markdown-it to deps and remove types package

[tats-u]

markdown-it-cjk-friendly has just supported (managed to avoid breaking changes in) markdown-it v15.