Bump postcss from 8.4.18 to 8.4.21

[dependabot[bot]]

Bumps postcss from 8.4.18 to 8.4.21.

Release notes

Sourced from postcss's releases.

8.4.21

• Fixed Input#error types (by @​hudochenkov).

8.4.20

• Fixed source map generation for childless at-rules like @layer.

8.4.19

• Fixed whitespace preserving after AST transformations (by @​romainmenke).

Changelog

Sourced from postcss's changelog.

8.4.21

• Fixed Input#error types (by Aleks Hudochenkov).

8.4.20

• Fixed source map generation for childless at-rules like @layer.

8.4.19

• Fixed whitespace preserving after AST transformations (by Romain Menke).

Commits

• 32ced0e Release 8.4.21 version
• a936b80 Update dependencies
• c9bd445 Fix types
• 4ad96d1 Merge pull request #1811 from hudochenkov/fix-input-types
• 15f5fad Add types for an Input.error()
• 43bd2df Merge pull request #1808 from hudochenkov/update-syntaxes
• 292b8f0 Add postcss-styled-syntax to list of syntaxes
• 29f5168 Update postcss-html and postcss-markdown repositories
• 1d4c509 Release 8.4.20 version
• 905082a Add Node.js 10 to CI
• Additional commits viewable in compare view

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