fix: upgrade prismjs from 1.19.0 to 1.20.0

Snyk has created this PR to upgrade prismjs from 1.19.0 to 1.20.0.

See this package in NPM:
https://www.npmjs.com/package/prismjs

See this project in Snyk:
https://app.snyk.io/org/philwareham/project/c8ae94e2-94bf-4092-bfad-1c3aa9f40249?utm_source=github&utm_medium=upgrade-pr

Author: snyk-bot

package.json

@@ -33,7 +33,7 @@
   "dependencies": {
     "createjs": "1.0.1",
     "lunr": "2.3.8",
-    "prismjs": "1.19.0",
+    "prismjs": "1.20.0",
     "textpattern-branding": "0.2.11"
   },
   "optionalDependencies": {