Update dependency rollup to v2.21.0

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
rollup (source)	devDependencies	minor	2.18.1 -> 2.21.0

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Release Notes

rollup/rollup

### [`v2.21.0`](https://togithub.com/rollup/rollup/blob/master/CHANGELOG.md#​2210) [Compare Source](https://togithub.com/rollup/rollup/compare/v2.20.0...v2.21.0) _2020-07-07_ ##### Features - Allow plugins to disable tree-shaking for individual modules to ensure even empty modules are associated with chunks ([#​3663](https://togithub.com/rollup/rollup/issues/3663)) ##### Pull Requests - [#​3663](https://togithub.com/rollup/rollup/pull/3663): Disable treeshaking per module ([@​lukastaegert](https://togithub.com/lukastaegert)) ### [`v2.20.0`](https://togithub.com/rollup/rollup/blob/master/CHANGELOG.md#​2200) [Compare Source](https://togithub.com/rollup/rollup/compare/v2.19.0...v2.20.0) _2020-07-06_ ##### Features - Support using a function to generate different chunk and asset naming patterns per chunk or asset ([#​3658](https://togithub.com/rollup/rollup/issues/3658)) - Add `referencedFiles` property to the chunk info in generateBundle to list referenced assets ([#​3661](https://togithub.com/rollup/rollup/issues/3661)) ##### Pull Requests - [#​3658](https://togithub.com/rollup/rollup/pull/3658): Add ability to use a function that returns a pattern string in all places where you could use a pattern string before ([@​frank-dspeed](https://togithub.com/frank-dspeed)) - [#​3661](https://togithub.com/rollup/rollup/pull/3661): Add referenced files to bundle ([@​lukastaegert](https://togithub.com/lukastaegert)) ### [`v2.19.0`](https://togithub.com/rollup/rollup/blob/master/CHANGELOG.md#​2190) [Compare Source](https://togithub.com/rollup/rollup/compare/v2.18.2...v2.19.0) _2020-07-05_ ##### Features - Allow plugins to return a Promise in the options hook ([#​3660](https://togithub.com/rollup/rollup/issues/3660)) ##### Pull Requests - [#​3660](https://togithub.com/rollup/rollup/pull/3660): Make options hooks async ([@​TomerAberbach](https://togithub.com/TomerAberbach)) ### [`v2.18.2`](https://togithub.com/rollup/rollup/blob/master/CHANGELOG.md#​2182) [Compare Source](https://togithub.com/rollup/rollup/compare/v2.18.1...v2.18.2) _2020-07-02_ ##### Bug Fixes - Do not remove spread element args when the corresponding positional parameter is unused ([#​3652](https://togithub.com/rollup/rollup/issues/3652)) ##### Pull Requests - [#​3652](https://togithub.com/rollup/rollup/pull/3652): Do not tree-shake arguments that contain a spread element ([@​lukastaegert](https://togithub.com/lukastaegert))

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