Update dependency postcss-custom-properties to v12

[renovate]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
postcss-custom-properties	^11.0.0 -> ^12.0.0

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Release Notes

csstools/postcss-plugins

v12.1.5

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• Add deprecation notice for importFrom and exportTo

see the discussion

v12.1.4

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• Fix .mjs in importFrom when using export default
• Fix .mjs in importFrom on Windows

v12.1.3

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• Reset plugin state after each process. It is now safe to use the plugin multiple times for different processes or when watching.

v12.1.2

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• Fix TypeScript transpilation.
• Avoid throwing errors on unexpected option objects.

v12.1.1

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• Fix Node 12/14 compatibility

v12.1.0

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• Add overrideImportFromWithRoot option
• Allow .mjs in importFrom
• Converted to typescript
• Correct typings for plugin options
• Fix unicode support in custom property names

v12.0.4

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• Fixed an issue that was causing synchronous mode to not being able to pick and transform properties that were added as part of the PostCSS flow. (#​132)

v12.0.3

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v12.0.2

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• Removed Sourcemaps from package tarball.
• Moved CLI to CLI Package. See announcement.

v12.0.1

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• Changed: now uses postcss-value-parser for parsing.
• Updated: documentation

v12.0.0

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• Updated: Support for PostCS 8+ (major).
• Updated: Support for Node 12+ (major).

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