fix: List actions inconsistent position

[afc163]

🤔 This is a ...

• New feature
• Bug fix
• Site / document update
• Component style update
• TypeScript definition update
• Refactoring
• Code style optimization
• Test Case
• Branch merge
• Other (about what?)

🔗 Related issue link

close #20648

💡 Background and solution

总结一下：

1. 当 List.Item 的 children 里只有一个节点，则始终开启 flex 模式，并使用 justify-content: space-between; 进行两端对齐。
2. 当 List.Item 的 children 里有多个节点，且一部分是 string，则不开启 flex 模式。右侧节点用 float: right 靠右。

📝 Changelog

Language	Changelog
🇺🇸 English	Fix List actions inconsistent position.
🇨🇳 Chinese	修复 List actions 位置不在右边的问题。

☑️ Self Check before Merge

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

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[codecov]

Codecov Report

Merging #20897 into 3.x-stable will not change coverage.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           3.x-stable   #20897   +/-   ##
===========================================
  Coverage       97.81%   97.81%           
===========================================
  Files             286      286           
  Lines            7749     7749           
  Branches         2183     2139   -44     
===========================================
  Hits             7580     7580           
  Misses            169      169

Impacted Files	Coverage Δ
components/list/Item.tsx	100% <100%> (ø)	⬆️

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