test: Format codebase with prettier

[eps1lon]

What:

Ensure codebase is formatted with prettier

Why:

Consistent formatting.

How:

Port testing-library/dom-testing-library#955

Checklist:

• [ ] Documentation added to the
  docs site
• Tests
• [ ] TypeScript definitions updated
• Ready to be merged
  

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[codecov]

Codecov Report

Merging #959 (79be437) into main (68fd459) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main      #959   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            4         4           
  Lines          140       140           
  Branches        28        28           
=========================================
  Hits           140       140           

Impacted Files	Coverage Δ
src/index.js	100.00% <100.00%> (ø)

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[github-actions]

🎉 This PR is included in version 12.1.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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