fix: Only move comments when extracting a struct from an enum variant
Motivating example:
```rs
#[derive(Debug, thiserror::Error)]
enum Error {
/// Some explanation for this error
#[error("message")]
$0Woops {
code: u32
}
}
```
now becomes
```rs
/// Some explanation for this error
#[derive(Debug, thiserror::Error)]
struct Woops{
code: u32
}
#[derive(Debug, thiserror::Error)]
enum Error {
#[error("message")]
Woops(Woops)
}
```
(the `thiserror::Error` derive being copied and the struct formatting aren't ideal, though those are issues for another day)
cf05b7db4d