Add babel plugin to handle class fields

Author: JoonasAapro

babel.config.json

@@ -20,6 +20,7 @@
         "corejs": false
       }
     ],
-    ["@babel/plugin-transform-regenerator", { "async": false }]
+    ["@babel/plugin-transform-regenerator", { "async": false }],
+    ["@babel/plugin-proposal-class-properties"]
   ]
 }

package-lock.json

@@ -15,6 +15,7 @@
     "@geoman-io/leaflet-geoman-free": "^2.18.3"
   },
   "devDependencies": {
+    "@babel/plugin-proposal-class-properties": "^7.18.6",
     "@decidim/dev": "^0.28.6",
     "@decidim/eslint-config": "^0.28.6",
     "@decidim/prettier-config": "^0.28.6",