ArrayAccess / Dynamic<Type> for typedef

I spend some time to integrate haxe <=> typescript and have a problem: haxe don't allow to specify typedef with a array access like:
```haxe
typedef A = {>Dynamic, otherField:Int }
```
Defining `interface` is not a good solution is most cases.

Typescript has many definition files (and we can convert them to haxe externs), so I think, support this feature is important.