chore(deps): update dependency postcss-custom-properties to v13

[renovate]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
postcss-custom-properties (source)	^12.1.10 -> ^13.1.0

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Release Notes

csstools/postcss-plugins

v13.1.0

Compare Source

• Added: Support for Cascade Layers.
• Improve plugin performance (port of fixes in v12.1.11)

v13.0.0

Compare Source

• Updated: Support for Node v14+ (major).
• Removed : importFrom feature (breaking).
• Removed : exportTo feature (breaking).
• Added support for local custom property declarations.

.example {
	--a-value: 20px;
	margin: var(--a-value);
}

/* becomes */

.example {
	--a-value: 20px;
	margin: 20px;
	margin: var(--a-value);
}

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