feat(design): TreeSelect add default placeholder and ConfigProvider support to config it global

[dengfuping]

📦 Modified package

• [x] @oceanbase/design
• [ ] @oceanbase/ui
• [ ] @oceanbase/icons
• [ ] @oceanbase/charts
• [ ] @oceanbase/util
• [ ] @oceanbase/codemod
• [ ] Other (about what?)

🤔 This is a ...

• [x] New feature
• [ ] Bug fix
• [ ] Site / documentation update
• [ ] Demo update
• [ ] Component style update
• [ ] TypeScript definition update
• [ ] Bundle size optimization
• [ ] Performance optimization
• [ ] Enhancement feature
• [ ] Internationalization
• [ ] Refactoring
• [ ] Code style optimization
• [ ] Test Case
• [ ] Branch merge
• [ ] Workflow
• [ ] Other (about what?)

🔗 Related issue link

• None

💡 Background and solution

📝 Changelog

Language	Changelog
🇺🇸 English
🇨🇳 Chinese	- 📢 TreeSelect 默认填充 placeholder，并支持通过 ConfigProvider locale.global.placeholder 或 locale.TreeSelect.placeholder 进行全局配置。- 🆕 ConfigProvider 新增 locale.TreeSelect.placeholder 属性，用于配置 Select 的默认 placeholder。

☑️ Self-Check before Merge

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

• [x] Doc is updated/provided or not needed
• [x] Demo is updated/provided or not needed
• [x] TypeScript definition is updated/provided or not needed
• [x] Tests is updated/provided or not needed
• [x] Changelog is provided or not needed

[github-actions[bot]]