Update dependency rollup to v2.26.6

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
rollup (source)	devDependencies	patch	2.26.5 -> 2.26.6

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Release Notes

rollup/rollup

### [`v2.26.6`](https://togithub.com/rollup/rollup/blob/master/CHANGELOG.md#​2266) [Compare Source](https://togithub.com/rollup/rollup/compare/v2.26.5...v2.26.6) _2020-08-27_ ##### Bug Fixes - Avoid conflicts between the namespace of synthetic named exports and local variables ([#​3747](https://togithub.com/rollup/rollup/issues/3747)) ##### Pull Requests - [#​3747](https://togithub.com/rollup/rollup/pull/3747): Properly deconflict synthetic named exports ([@​lukastaegert](https://togithub.com/lukastaegert))

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