feat(Textarea): support count API

[RylanBot]

🤔 这个 PR 的性质是？

• 日常 bug 修复
• 新特性提交
• 文档改进
• 演示代码改进
• 组件样式/交互改进
• CI/CD 改进
• 重构
• 代码风格优化
• 测试用例
• 分支合并
• 其他

🔗 相关 Issue

💡 需求背景和解决方案

• feat(Textarea): support count API TDesignOteam/tdesign-api#785

📝 更新日志

• feat(Textarea): 支持 count API，用于自定义计数元素的渲染

• 本条 PR 不需要纳入 Changelog

☑️ 请求合并前的自查清单

⚠️ 请自检并全部勾选全部选项。⚠️

• 文档已补充或无须补充
• 代码演示已提供或无须提供
• TypeScript 定义已补充或无须补充
• Changelog 已提供或无须提供

[pkg-pr-new]

• tdesign-react-demo

  npm i https://pkg.pr.new/tdesign-react@4003
  

commit: 4bd1c04

[github-actions]

[Copilot]

Pull request overview

This PR adds a new count API to the Textarea component, allowing developers to customize how the character counter is rendered.

Key Changes:

• Added count prop that accepts boolean or a custom render function with context parameters (value, count, maxLength, maxCharacter)
• Updated example to demonstrate custom count rendering with styled elements
• Modified internal logic to conditionally show/hide and render count based on the new count prop

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
packages/components/textarea/type.ts	Added count prop type definition to support boolean or custom render function
packages/components/textarea/textarea.md	Updated Chinese documentation to include the new count API
packages/components/textarea/textarea.en-US.md	Updated English documentation with the new count prop description
packages/components/textarea/_example/maxlength.tsx	Added example demonstrating custom count rendering with inline styles
packages/components/textarea/Textarea.tsx	Implemented count logic with conditional rendering based on prop value
test/snap/snapshots/ssr.test.jsx.snap	Updated SSR snapshot to reflect custom count rendering in example
test/snap/snapshots/csr.test.jsx.snap	Updated CSR snapshot to reflect custom count rendering in example

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