fix(components): [dropdown] improve types

[ModyQyW]

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This PR improves dropdown types.

Previously, we have to write types to use dropdown ref to avoid any.

const dropdown1 = ref()
import { ElDropdown } from 'element-plus'
const dropdown1 = ref<InstanceType<typeof ElDropdown>>()

After this, we can directly import and use the type.

const dropdown1 = ref()
import type { DropdownInstance } from 'element-plus'
const dropdown1 = ref<DropdownInstance>()

Also, I updated the example file to use the new type.

[pull-request-triage]

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[github-actions]

[github-actions]

Hello @ModyQyW, thank you for contributing to element-plus, please see our guideline to see how to make contribution

[github-actions]

🧪 Playground Preview: https://element-plus.run/?pr=12514
Please comment the example via this playground if needed.

[btea]

@ModyQyW Thank you for your contribution ❤️