chore: bump react-shadow from 19.1.0 to 20.4.0 (#894)

Bumps [react-shadow](https://github.com/Wildhoney/ReactShadow) from 19.1.0 to 20.4.0.
- [Commits](Wildhoney/ReactShadow@v19.1.0...v20.4.0)

---
updated-dependencies:
- dependency-name: react-shadow
  dependency-type: direct:development
  update-type: version-update:semver-major
...

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