WebUI: migrate stylelint config file format

From https://stylelint.io/user-guide/configure :
>Stylelint currently supports other configuration locations and formats, but we may remove these
>in the future:
>...
>.stylelintrc.json file

src/webui/www/.stylelintrc.json

@@ -1,19 +0,0 @@
-{
-  "extends": "stylelint-config-standard",
-  "plugins": [
-    "stylelint-order"
-  ],
-  "ignoreFiles": ["private/css/lib/*.css"],
-  "rules": {
-    "color-hex-length": null,
-    "comment-empty-line-before": null,
-    "comment-whitespace-inside": null,
-    "function-name-case": null,
-    "length-zero-no-unit": null,
-    "no-descending-specificity": null,
-    "order/properties-alphabetical-order": true,
-    "selector-class-pattern": null,
-    "selector-id-pattern": null,
-    "value-keyword-case": null
-  }
-}

src/webui/www/stylelint.config.mjs

@@ -0,0 +1,19 @@
+export default {
+    extends: "stylelint-config-standard",
+    plugins: [
+        "stylelint-order"
+    ],
+    ignoreFiles: ["private/css/lib/*.css"],
+    rules: {
+        "color-hex-length": null,
+        "comment-empty-line-before": null,
+        "comment-whitespace-inside": null,
+        "function-name-case": null,
+        "length-zero-no-unit": null,
+        "no-descending-specificity": null,
+        "order/properties-alphabetical-order": true,
+        "selector-class-pattern": null,
+        "selector-id-pattern": null,
+        "value-keyword-case": null
+    }
+};