Set experimentalDecorators to true for Deno decorators support

[fecony]

Deno 1.40 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:

{
  "compilerOptions": {
    "experimentalDecorators": true
  }
}