wit-parser: Doc comments on interface imports/export world items are discarded

```wit
interface i {}

world w {
/// this is lost
import i;
}
```

These comments are discarded during parsing, meaning e.g. [this](https://github.com/WebAssembly/wasi-http/blob/ed620562ee1d089a20aa67cd7790782d03f2f505/wit/proxy.wit#L44-L47) doesn't survive a round-trip through binary WIT encoding.

I spent a little time trying to fix this but I think it needs a breaking change to the package metadata serialization format.