chore: bump @babel/plugin-proposal-nullish-coalescing-operator

Bumps [@babel/plugin-proposal-nullish-coalescing-operator](https://github.com/babel/babel/tree/HEAD/packages/babel-plugin-proposal-nullish-coalescing-operator) from 7.12.13 to 7.13.8.
- [Release notes](https://github.com/babel/babel/releases)
- [Changelog](https://github.com/babel/babel/blob/main/CHANGELOG.md)
- [Commits](https://github.com/babel/babel/commits/v7.13.8/packages/babel-plugin-proposal-nullish-coalescing-operator)

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