Bump prismjs from 1.20.0 to 1.21.0 (#47)

Bumps [prismjs](https://github.com/PrismJS/prism) from 1.20.0 to 1.21.0.
- [Release notes](https://github.com/PrismJS/prism/releases)
- [Changelog](https://github.com/PrismJS/prism/blob/master/CHANGELOG.md)
- [Commits](PrismJS/prism@v1.20.0...v1.21.0)

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