fix: upgrade rollup from 2.27.1 to 2.28.1

Snyk has created this PR to upgrade rollup from 2.27.1 to 2.28.1.

See this package in npm:
https://www.npmjs.com/package/rollup

See this project in Snyk:
https://app.snyk.io/org/dmaccormack/project/51e346e8-0429-4dc8-902a-93e77d72b41c?utm_source=github&utm_medium=upgrade-pr

Author: snyk-bot

package-lock.json

@@ -50,7 +50,7 @@
     "jest": "^26.4.2",
     "jsdoc": "^3.6.6",
     "prettier": "^2.1.2",
-    "rollup": "^2.27.1",
+    "rollup": "^2.28.1",
     "rollup-plugin-eslint": "^7.0.0",
     "rollup-plugin-terser": "^7.0.2"
   },