[Test] Add PostCSS syntaxes for embedded styles

(required since Stylelint 14.0.0)

.stylelintrc.json

@@ -1,5 +1,15 @@
 {
     "extends": "stylelint-config-standard",
+    "overrides": [
+        {
+            "files": ["**/*.html"],
+            "customSyntax": "postcss-html"
+        },
+        {
+            "files": ["**/*.js"],
+            "customSyntax": "@stylelint/postcss-css-in-js"
+        }
+    ],
     "ignoreFiles": [
         "**/*.min.css",
         "**/*.min.js",

package.json

@@ -2,7 +2,9 @@
     "devDependencies": {
         "eslint": "*",
         "stylelint": ">=13.6.0",
-        "stylelint-config-standard": "*"
+        "stylelint-config-standard": "*",
+        "postcss-html": "*",
+        "@stylelint/postcss-css-in-js": "*"
     },
     "eslintIgnore": [
         "*.min.js",