fix: expandable.expandIconColumnIndex setting failure in version 4.0

[fomenyesu]

🤔 This is a ...

• New feature
• Bug fix
• Site / document update
• Component style update
• TypeScript definition update
• Refactoring
• Code style optimization
• Test Case
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🔗 Related issue link

#17942

💡 Background and solution

4.0版本下的 expandable.expandIconColumnIndex 设置会被覆盖掉。3.x版本没有这个问题。对比源码发现3.x版本判断了用户设置里是否自定义里expandIconColumnIndex，而4.0版本expandIconColumnIndex设置方式有误，没有做判断。会覆盖掉用户设置的expandIconColumnIndex。

3.x 代码片段：

    let expandIconColumnIndex = columns[0] && columns[0].key === 'selection-column' ? 1 : 0;
    if ('expandIconColumnIndex' in restProps) {
      expandIconColumnIndex = restProps.expandIconColumnIndex as number;
    }

4.0 代码片段：

  // ========================== Expandable ==========================
  const mergedExpandable: ExpandableConfig<RecordType> = {
    ...expandable,
  };
...
  // Adjust expand icon index
  if (expandType === 'nest') {
    mergedExpandable.expandIconColumnIndex = rowSelection ? 1 : 0;
  }

修改后：

  // ========================== Expandable ==========================
  const mergedExpandable: ExpandableConfig<RecordType> = {
    ...expandable,
  };
...
  // Adjust expand icon index, no overwrite expandIconColumnIndex if set.
  if (expandType === 'nest' && !('expandIconColumnIndex' in mergedExpandable)) {
    mergedExpandable.expandIconColumnIndex = rowSelection ? 1 : 0;
  }

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• 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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