Bump prismjs from 1.20.0 to 1.21.0

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)

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

Author: dependabot[bot]

package.json

@@ -33,7 +33,7 @@
   "dependencies": {
     "easeljs": "1.0.2",
     "preloadjs": "1.0.1",
-    "prismjs": "1.20.0",
+    "prismjs": "1.21.0",
     "textpattern-branding": "0.2.11",
     "tweenjs": "1.0.2"
   },