Allow custom `DECL` TypeScript definition string

Sometimes your type might look like this:


```rs
pub struct Color {
  r: i8,
  g: i8,
  b: i8,
}
```

but you want a custom `serde` impl + string representation in TypeScript:

```ts
// So it can look like #abcdef
export type Color = `#${string}{string}{string}`
```