Feature request: configuration option `validAtRules` (for `UnknownAtRules`), just like the existing `validProperties` (for `UnknownProperty`)

[danielweck]

Code references:

vscode-css-languageservice/src/services/lintRules.ts

Lines 53 to 55 in 06085e8

	export const Settings = {
	ValidProperties: new Setting('validProperties', localize('rule.validProperties', "A list of properties that are not validated against the `unknownProperties` rule."), [])
	};

vscode-css-languageservice/src/services/lintRules.ts

Lines 43 to 44 in 06085e8

	UnknownProperty: new Rule('unknownProperties', localize('rule.unknownProperty', "Unknown property."), Warning),
	UnknownAtRules: new Rule('unknownAtRules', localize('rule.unknownAtRules', "Unknown at-rule."), Warning),

Related issues:
#113
microsoft/typescript-styled-plugin#83
microsoft/typescript-styled-plugin#82
microsoft/typescript-styled-plugin#77

Related documentation:
https://github.com/microsoft/typescript-styled-plugin#validproperties

[aeschli]

Note that in VSCode this can already be solved by adding custom data: https://code.visualstudio.com/Docs/languages/CSS#_css-custom-data