Support default value in decorators

```
extern dec encode(target: unknown, encoding: string, encodedAs?: string | bytes = string);
```
Would be good to be able to do that.

To decide in design is do we want the property with default to be marked optional or not

```
extern dec encode(target: unknown, encoding: string, encodedAs?: string | bytes = string);
// vs
extern dec encode(target: unknown, encoding: string, encodedAs: string | bytes = string);
```

In typescript they have to be marked required