chore(deps): 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] <[email protected]>

Author: dependabot[bot]

package-lock.json

@@ -49,7 +49,7 @@
     "phoenix": "1.4.13",
     "preact": "8.3.1",
     "preact-compat": "3.18.4",
-    "prismjs": "1.15.0",
+    "prismjs": "1.21.0",
     "query-string": "6.2.0",
     "react": "16.9.0",
     "react-dom": "16.9.0",