feat(dropdown): allow pass string for prop trigger

[CarterLi]

This is a ...

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

What's the background?

点击展开下拉框是非常常见的需求（个人感觉比hover展开还常见），但是现在dropdown组件强制要求传入数组而不能直接传入一个字符串。

我试过了dropdown组件实际上传入字符串是可以正常工作的，但不知为何在prop声明的时候没有允许传入字符串

https://github.com/vueComponent/ant-design-vue/blob/next/components/vc-trigger/Trigger.jsx#L46

另外传入字符串比传入数组性能更好。因为数组是对象，每次vdom diff时数组的引用都是不同的，可能导致watch等频繁触发，除非把数组定义在data里面，而在模板里使用变量。

API Realization (Optional if not new feature)

<a-dropdown trigger="click" />

而不是

<a-dropdown :trigger="['click']" />

What's the effect? (Optional if not new feature)

允许trigger属性直接指定字符串而不是字符串数组

Changelog description (Optional if not new feature)

1. English description

feat(dropdown): allow pass string for prop trigger

2. Chinese description (optional)

feat(dropdown): 允许trigger属性直接指定字符串

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

[github-actions]

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