refactor: convert test setup to Typescript

[ph-fritsche]

What:

Convert the __test__/helpers/utils to Typescript.

Why:

We want to convert the whole codebase to Typescript because using typed code is easier and less error-prone.

How:

const { element } = setup(`<div></div>`) // element is HTMLElement

const { element } = setup<HTMLDivElement>(`<div></div>`) // element is HTMLDivElement

const { elements: [first, second] } = setup<[HTMLInputElement, HTMLTextareaElement]>(`<input/><textarea></textarea>`)
// first is HTMLInputElement, second is HTMLTextareaElement

This allowed to remove type casts and optional chains.

Checklist:

• [N/A] Documentation
• Tests
• Ready to be merged

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[github-actions]

🎉 This PR is included in version 13.3.0 🎉

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