build: update rollup to version ~2.28.0

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
rollup (source)	devDependencies	minor	~2.27.0 -> ~2.28.0

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Release Notes

rollup/rollup

v2.28.1

Compare Source

2020-09-21

Bug Fixes

• Fix a path slash issue when using the preserveModulesRoot option on Windows (#​3791)

Pull Requests

• #​3791: Fix preserveModulesRoot path on Windows (@​lukastaegert)

v2.28.0

Compare Source

2020-09-21

Features

• Add an option to treat modules at a given path as located at root when preserving modules (#​3786)

Pull Requests

• #​3786: Add preserveModulesRoot config option (@​davidroeca)

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