move src/core/css to libs/css and add alias to tsconfig

src/Umbraco.Web.UI.Client/tsconfig.json

@@ -20,6 +20,7 @@
 		"resolveJsonModule": true,
 		"baseUrl": ".",
 		"paths": {
+			"@umbraco-cms/css": ["libs/css/custom-properties.css"],
 			"@umbraco-cms/models": ["libs/models"],
 			"@umbraco-cms/backend-api": ["libs/backend-api"],
 			"@umbraco-cms/context-api": ["libs/context-api"],

src/Umbraco.Web.UI.Client/web-test-runner.config.mjs

@@ -12,6 +12,7 @@ export default {
 				importMap: {
 					imports: {
 						'src/': './src/',
+						'@umbraco-cms/css': './libs/css/custom-properties.css',
 						'@umbraco-cms/models': './libs/models/index.ts',
 						'@umbraco-cms/backend-api': './libs/backend-api/index.ts',
 						'@umbraco-cms/context-api': './libs/context-api/index.ts',