Feature: Do not require `#[ts(export)]` if `#[ts(export_to = )]` is present

[stegaBOB]

I'm not sure if there's a reason for requiring #[ts(export)] on all structs as well as export_to. I have to provide both attributes every time, which is a bit annoying. I'd be happy to implement a quick fix if removing that requirement is acceptable. Would be a breaking change though, but probably fine since it would only need a minor bump pre 1.0.

[stegaBOB]

Should have taken a closer look at the code before opening this issue. I can see the difference for why there would be separate options. I think allowing a default like in #138 would probably fix the ergonomics (I commented on that issue too!). Closing this now.