Bump rollup from 2.26.11 to 2.28.1

Bumps [rollup](https://github.com/rollup/rollup) from 2.26.11 to 2.28.1.
- [Release notes](https://github.com/rollup/rollup/releases)
- [Changelog](https://github.com/rollup/rollup/blob/master/CHANGELOG.md)
- [Commits](rollup/rollup@v2.26.11...v2.28.1)

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

package.json

@@ -128,7 +128,7 @@
     "npm-run-all": "^4.1.5",
     "popper.js": "^1.16.1",
     "postcss-cli": "^7.1.2",
-    "rollup": "^2.26.11",
+    "rollup": "^2.28.1",
     "rollup-plugin-istanbul": "^2.0.1",
     "shelljs": "^0.8.4",
     "sirv-cli": "^1.0.6",