Bump prismjs from 1.15.0 to 1.21.0

Bumps [prismjs](https://github.com/PrismJS/prism) from 1.15.0 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@v1.15.0...v1.21.0)

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

package.json

@@ -33,7 +33,7 @@
     "lodash": "^4.17.11",
     "medium-editor": "^5.23.3",
     "polished": "^2.3.0",
-    "prismjs": "^1.15.0",
+    "prismjs": "^1.21.0",
     "prop-types": "^15.6.2",
     "react": "^16.9.0",
     "react-dom": "^16.9.0",