Update dependency rollup to v2.28.1

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
rollup (source)	devDependencies	minor	2.27.1 -> 2.28.1

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Release Notes

rollup/rollup

### [`v2.28.1`](https://togithub.com/rollup/rollup/blob/master/CHANGELOG.md#​2281) [Compare Source](https://togithub.com/rollup/rollup/compare/v2.28.0...v2.28.1) _2020-09-21_ ##### Bug Fixes - Fix a path slash issue when using the preserveModulesRoot option on Windows ([#​3791](https://togithub.com/rollup/rollup/issues/3791)) ##### Pull Requests - [#​3791](https://togithub.com/rollup/rollup/pull/3791): Fix preserveModulesRoot path on Windows ([@​lukastaegert](https://togithub.com/lukastaegert)) ### [`v2.28.0`](https://togithub.com/rollup/rollup/blob/master/CHANGELOG.md#​2280) [Compare Source](https://togithub.com/rollup/rollup/compare/v2.27.1...v2.28.0) _2020-09-21_ ##### Features - Add an option to treat modules at a given path as located at root when preserving modules ([#​3786](https://togithub.com/rollup/rollup/issues/3786)) ##### Pull Requests - [#​3786](https://togithub.com/rollup/rollup/pull/3786): Add preserveModulesRoot config option ([@​davidroeca](https://togithub.com/davidroeca))

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