feat: integrate Mermaid plugin for ByteMD editor

[Copilot]

Adds Mermaid diagram support to the ByteMD editor, enabling users to render flowcharts, sequence diagrams, and other Mermaid visualizations directly in markdown.

Changes

• Added @bytemd/plugin-mermaid@^1.22.0 dependency
• Registered mermaid plugin alongside existing plugins (gfm, math, breaks)

Usage

```mermaid
graph TD
A[开始] --> B{判断}
B -->|是| C[执行方案一]
B -->|否| D[执行方案二]

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<details>

<summary>Original prompt</summary>

> 
> ----
> 
> *This section details on the original issue you should resolve*
> 
> <issue_title>功能建议 给ByteMD集成Mermaid插件</issue_title>
> <issue_description>给ByteMD集成Mermaid插件的过程并不复杂，核心是正确安装插件并在编辑器和查看器中配置。下面是一个汇总了所有必要依赖和步骤的指南，能帮助你快速完成集成。
> 
> ### 📚 所需依赖包一览
> 
> 开始之前，你的项目需要安装以下依赖：
> 
> <table><thead><tr><th><span>依赖类型</span></th><th><span>包名</span></th><th><span>说明</span></th></tr></thead><tbody><tr><td><span>核心依赖</span></td><td><code style="white-space:pre;font-family:SFMono-Regular,Consolas,Monaco,monospace">bytemd</code></td><td><span>ByteMD的核心库</span><a href="https://blog.csdn.net/gitblog_00041/article/details/138146704" target="_blank" rel="noreferrer" style="cursor: default;"></a></td></tr></tbody></table>
> 
> 。框架适配`@bytemd/react`用于React项目的组件[https://blog.csdn.net/gitblog_00041/article/details/138146704](https://blog.csdn.net/gitblog_00041/article/details/138146704)
> 
> 。（根据你的框架选择）`@bytemd/vue`用于Vue 2项目的组件[https://developer.aliyun.com/article/900370](https://developer.aliyun.com/article/900370)
> 
> 。`@bytemd/vue-next`用于Vue 3项目的组件[https://developer.aliyun.com/article/900370](https://developer.aliyun.com/article/900370)
> 
> 。**关键插件**`@bytemd/plugin-mermaid`**实现Mermaid图表功能的核心插件**[https://juejin.cn/post/7230339089617125434](https://juejin.cn/post/7230339089617125434)
> 
> 。辅助插件`@bytemd/plugin-gfm`支持GitHub风格的Markdown语法（如表格、删除线）[https://blog.csdn.net/gitblog_00041/article/details/138146704](https://blog.csdn.net/gitblog_00041/article/details/138146704)
> 
> 。样式文件`bytemd/dist/index.css`ByteMD的基础样式[https://blog.csdn.net/gitblog_00041/article/details/138146704](https://blog.csdn.net/gitblog_00041/article/details/138146704)
> 
> <table><tbody><tr><td><span>。</span></td></tr></tbody></table>
> 
> ### 🛠️ 集成步骤（以React为例）
> 
> 下面的步骤以React项目为例，其他框架思路类似。
> 
> 1.  **安装依赖**
>     在项目根目录下，通过npm或yarn安装所有必需的包。
>     
>     
>     
> 
> ```
> # 使用 npm
> npm install bytemd @bytemd/react @bytemd/plugin-mermaid @bytemd/plugin-gfm
> 
> # 或使用 yarn
> yarn add bytemd @bytemd/react @bytemd/plugin-mermaid @bytemd/plugin-gfm
> ```
> 
> **导入组件和插件**
> 在你的React组件文件中，导入所需的模块。
> 
> javascript
> 
> ```
> import React, { useState } from 'react';
> // 导入Editor组件
> import { Editor } from '@bytemd/react';
> // 导入Mermaid和GFM插件
> import mermaid from '@bytemd/plugin-mermaid';
> import gfm from '@bytemd/plugin-gfm';
> // 导入基础样式
> import 'bytemd/dist/index.css';
> ```
> 
> **配置插件**
> 初始化插件并放入一个数组中。
> 
> javascript
> 
> ```
> // 配置插件
> const plugins = [
>   gfm(),   // 启用GFM扩展语法
>   mermaid(), // 启用Mermaid图表支持
>   // 你可以继续添加其他插件...
> ];
> ```
> 
> **使用组件**
> 在组件中使用`Editor`，并将插件数组和内容状态绑定上去。
> 
> javascript
> 
> ```
> const App = () => {
>   // 使用state来保存Markdown内容
>   const [value, setValue] = useState('');
> 
>   return (
>     <Editor
>       value={value}
>       plugins={plugins}
>       onChange={(v) => setValue(v)}
>     />
>   );
> };
> 
> export default App;
> ```
> 
> 完成以上步骤后，你的ByteMD编辑器就已经支持Mermaid图表了。你可以尝试在编辑器中输入以下Mermaid代码进行测试：
> 
> markdown
> 
> \`\`\`mermaid graph TD A\[开始\] --> B{判断} B -->|是| C\[执行方案一\] B -->|否| D\[执行方案二\] C --> E\[结束\] D --> E \`\`\`</issue_description>
> 
> ## Comments on the Issue (you are @copilot in this section)
> 
> <comments>
> </comments>
> 


</details>

- Fixes halo-sigs/plugin-bytemd#44

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