Add loose mode and env config to .swcrc for proper class transforms

The issue was that .swcrc was missing critical SWC configuration that
Shakapacker's swc-loader normally provides (loose: true and env config).
When .swcrc exists, it takes precedence over the loader config, so we
need to include these settings explicitly.

This fixes Stimulus controller static properties not being properly
transpiled, which was causing the RSpec tests to fail.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: justin808

.swcrc

@@ -9,9 +9,14 @@
       "react": {
         "runtime": "automatic"
       }
-    }
+    },
+    "loose": true
   },
   "module": {
     "type": "es6"
+  },
+  "env": {
+    "coreJs": 3,
+    "mode": "entry"
   }
 }