chore(deps): update rollup to v2.1.0

[thi-bot]

This PR contains the following updates:

Package	Update	Type	Change	Sourcegraph
rollup (source)	minor	devDependencies	2.0.6 -> 2.1.0

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Release Notes

rollup/rollup

### [`v2.1.0`](https://togithub.com/rollup/rollup/blob/master/CHANGELOG.md#​210) [Compare Source](https://togithub.com/rollup/rollup/compare/v2.0.6...v2.1.0) _2020-03-18_ ##### Features - Allow specifying an importer when emitting files to resolve relative ids ([#​3442](https://togithub.com/rollup/rollup/issues/3442)) ##### Pull Requests - [#​3442](https://togithub.com/rollup/rollup/pull/3442): Add optional `importer` parameter to `this.emitFile` ([@​lukastaegert](https://togithub.com/lukastaegert))

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