Bump rollup from 2.20.0 to 2.21.0

Bumps [rollup](https://github.com/rollup/rollup) from 2.20.0 to 2.21.0.
- [Release notes](https://github.com/rollup/rollup/releases)
- [Changelog](https://github.com/rollup/rollup/blob/master/CHANGELOG.md)
- [Commits](https://github.com/rollup/rollup/compare/v2.20.0...v2.21.0)

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

package.json

@@ -104,7 +104,7 @@
     "postcss": "^7.0.27",
     "postcss-selector-prepend": "^0.5.0",
     "request": "^2.88.0",
-    "rollup": "~2.20.0",
+    "rollup": "~2.21.0",
     "rollup-plugin-includepaths": "~0.2.3",
     "rollup-plugin-progress": "^1.1.1",
     "rollup-plugin-visualizer": "~4.0.4",