Update dependency postcss to v7.0.18

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
postcss (source)	devDependencies	patch	7.0.17 -> 7.0.18

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Release Notes

postcss/postcss

### [`v7.0.18`](https://togithub.com/postcss/postcss/blob/master/CHANGELOG.md#​7018) [Compare Source](https://togithub.com/postcss/postcss/compare/7.0.17...7.0.18) - Fix TypeScript type definitions (by Jan Buschtöns).

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