Set experimentalDecorators to true for Deno decorators support

[Deno 1.40](https://deno.com/blog/v1.40) introduced support for TC39 stage 3 decorators. Causing following issue:

From mentioned release blog post:
> If you rely on the legacy “experimental TypeScript decorators”, you can still use them with the following configuration:

```json 
{
  "compilerOptions": {
    "experimentalDecorators": true
  }
}
```

deno.json

@@ -10,6 +10,7 @@
     }
   },
   "compilerOptions": {
+    "experimentalDecorators": true,
     "emitDecoratorMetadata": true
   },
   "importMap": "./import_map.json",