chore(deps-dev): bump postcss-loader in /src/packages/excalidraw (#3585)
Bumps [postcss-loader](https://github.com/webpack-contrib/postcss-loader) from 5.2.0 to 5.3.0.
- [Release notes](https://github.com/webpack-contrib/postcss-loader/releases)
- [Changelog](https://github.com/webpack-contrib/postcss-loader/blob/master/CHANGELOG.md)
- [Commits](https://github.com/webpack-contrib/postcss-loader/compare/v5.2.0...v5.3.0)
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