feat: improve footnote support with better styling and documentation

[vincent067]

Summary

This PR improves the footnote support in the WeChat Markdown Editor with better styling and comprehensive documentation.

Changes

Technical Improvements

• Refactored footnotes.ts: Replaced module-level Map with WeakMap for better render context isolation, preventing potential issues with concurrent renderings
• Improved regex patterns: Enhanced footnote definition matching to support both numeric [^1] and named [^note] references more robustly
• Better whitespace handling: Added .trim() to footnote content for cleaner output
• WeChat-compatible styling: Added inline styles optimized for WeChat's rendering engine

Documentation

• Added footnote feature to the core features list in README
• Created new "扩展语法" section with detailed footnote usage examples
• Documented both numeric and named footnote syntax
• Explained the click-to-jump functionality

Usage Example

微信 Markdown 编辑器[^1] 是一款高度简洁的编辑器[^note]。

[^1]: https://github.com/doocs/md
[^note]: 支持自定义主题样式、内容管理、多图床、AI 助手等特性。

Testing

• Footnote references render correctly as superscript
• Footnote definitions compile into a numbered list
• Click-to-jump links work properly
• Styling is compatible with WeChat's rendering

Related Issue

Closes #1334

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Thanks for maintaining this awesome project!

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[yanglbme]

项目原本就支持了脚注，你改了之后，好像反而有点问题？