feat(plugin): add custom styles, icons, and class names for context-menu plugin

[mealcomes]

[中文版模板 / Chinese template]

🤔 This is a ...

• New feature
• Bug fix
• TypeScript definition update
• Bundle size optimization
• Performance optimization
• Enhancement feature
• Refactoring
• Update dependency
• Code style optimization
• Test Case
• Branch merge
• Site / documentation update
• Demo update
• Workflow
• Chore
• Release
• Other (about what?)

🔗 Related issue link

related: #2032 #4151

💡 Background and solution

基于右键菜单插件 (context-menu) 无法自定义类名和样式的问题，进行配置的更新，具体如下：

• 菜单样式和类名通过 CustomMenuAttributions 统一配置
• 图标类型说明
• iconName: 'copy' — 内置 emoji 图标（兼容原版本）
• customIcon: { svg: '...', width: 16, height: 16 } — SVG 图标
• customIcon: (menuItem) => HTMLElement — 渲染函数
• 自定义类名说明
• CustomMenuAttributions.class — 统一追加类名
• MenuItem.customClassName — 单项精细化类名

具体使用示例见 context-menu.ts

📝 Changelog

Language	Changelog
🇺🇸 English	add custom styles, icons, and class names for context-menu plugin
🇨🇳 Chinese	为右键菜单插件添加自定义样式、图标和类名称

☑️ Self-Check before Merge

⚠️ Please check all items below before requesting a reviewing. ⚠️

• Doc is updated/provided or not needed
• Demo is updated
• TypeScript definition is updated
• Changelog is provided or not needed

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🚀 Summary

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🔍 Walkthrough

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