feature: select dropdown for enum as like union type supports it

Hi!

For ts union type, it generates select dropdown ui,

```ts
type GenderType = "male" | "female"
```

while enum does not.

```ts
enum Gender {
  Male = "male",
  Female = "female",
}
```

So I had to create corresponding union type from existing enum.
I think supporting enum for dropdown would be good.

What do you think?