Bump @babel/plugin-proposal-decorators from 7.4.4 to 7.6.0 (#12232)

Bumps [@babel/plugin-proposal-decorators](https://github.com/babel/babel) from 7.4.4 to 7.6.0.
- [Release notes](https://github.com/babel/babel/releases)
- [Changelog](https://github.com/babel/babel/blob/master/CHANGELOG.md)
- [Commits](https://github.com/babel/babel/compare/v7.4.4...v7.6.0

)

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