fix(deps): update dependency postcss-mixins to v6

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
postcss-mixins	dependencies	major	^4.0.1 -> ^6.0.0

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Release Notes

postcss/postcss-mixins

### [`v6.2.3`](https://togithub.com/postcss/postcss-mixins/blob/master/CHANGELOG.md#​623) [Compare Source](https://togithub.com/postcss/postcss-mixins/compare/6.2.2...6.2.3) - Fix plugin to work with webpack watch (by [@​Mesqalito](https://togithub.com/Mesqalito)). ### [`v6.2.2`](https://togithub.com/postcss/postcss-mixins/blob/master/CHANGELOG.md#​622) [Compare Source](https://togithub.com/postcss/postcss-mixins/compare/6.2.1...6.2.2) - Fix source map for mixins from SugarSS and CSS files. ### [`v6.2.1`](https://togithub.com/postcss/postcss-mixins/blob/master/CHANGELOG.md#​621) [Compare Source](https://togithub.com/postcss/postcss-mixins/compare/6.2.0...6.2.1) - Update `sugarss` and `postcss-simple-vars` (by [@​ambar](https://togithub.com/ambar)). ### [`v6.2.0`](https://togithub.com/postcss/postcss-mixins/compare/6.1.1...6.2.0) [Compare Source](https://togithub.com/postcss/postcss-mixins/compare/6.1.1...6.2.0) ### [`v6.1.1`](https://togithub.com/postcss/postcss-mixins/blob/master/CHANGELOG.md#​611) [Compare Source](https://togithub.com/postcss/postcss-mixins/compare/6.1.0...6.1.1) - Add `parent` to `dependency` message (by Keisuke Kan). - Throw error on wrong mixin type. ### [`v6.1.0`](https://togithub.com/postcss/postcss-mixins/compare/6.0.1...6.1.0) [Compare Source](https://togithub.com/postcss/postcss-mixins/compare/6.0.1...6.1.0) ### [`v6.0.1`](https://togithub.com/postcss/postcss-mixins/blob/master/CHANGELOG.md#​601) [Compare Source](https://togithub.com/postcss/postcss-mixins/compare/6.0.0...6.0.1) - Fix multiple `@mixin-content` usage. ### [`v6.0.0`](https://togithub.com/postcss/postcss-mixins/compare/5.4.1...6.0.0) [Compare Source](https://togithub.com/postcss/postcss-mixins/compare/5.4.1...6.0.0) ### [`v5.4.1`](https://togithub.com/postcss/postcss-mixins/blob/master/CHANGELOG.md#​541) [Compare Source](https://togithub.com/postcss/postcss-mixins/compare/5.4.0...5.4.1) - Fix support nested function mixins (by Lam Tran). ### [`v5.4.0`](https://togithub.com/postcss/postcss-mixins/compare/5.3.0...5.4.0) [Compare Source](https://togithub.com/postcss/postcss-mixins/compare/5.3.0...5.4.0) ### [`v5.3.0`](https://togithub.com/postcss/postcss-mixins/compare/5.2.0...5.3.0) [Compare Source](https://togithub.com/postcss/postcss-mixins/compare/5.2.0...5.3.0) ### [`v5.2.0`](https://togithub.com/postcss/postcss-mixins/compare/5.1.0...5.2.0) [Compare Source](https://togithub.com/postcss/postcss-mixins/compare/5.1.0...5.2.0) ### [`v5.1.0`](https://togithub.com/postcss/postcss-mixins/compare/5.0.1...5.1.0) [Compare Source](https://togithub.com/postcss/postcss-mixins/compare/5.0.1...5.1.0) ### [`v5.0.1`](https://togithub.com/postcss/postcss-mixins/blob/master/CHANGELOG.md#​501) [Compare Source](https://togithub.com/postcss/postcss-mixins/compare/5.0.0...5.0.1) - Use globby 6 and PostCSS 5.1. ### [`v5.0.0`](https://togithub.com/postcss/postcss-mixins/compare/4.0.2...5.0.0) [Compare Source](https://togithub.com/postcss/postcss-mixins/compare/4.0.2...5.0.0)

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