wit-parser: Add validation hooks for custom linting

[wilfreddenton]

#2418

There are two strategies I've explored for giving consumers access to source locations:

1. Make spans available on resolved items so consumers can inspect them after resolution. There are two ways I found to do this

   1. Spans on structs (wit-parser-spans-in-structs) - Add span: Option<Span> directly to TypeDef, Function, Interface, World
      • Pros: Direct access pattern; spans travel with the data automatically during merges
      • Cons: Breaking change to public struct layouts
   2. Spans in HashMaps (wit-parser-spans-in-resolve) - Store spans in HashMap<Id, Span> fields on Resolve
      • Pros: No changes to existing struct layouts (Resolve derives Default so not likely to break consumers)
      • Cons: Indirect lookup; requires explicit remapping during merges

2. Hook into internal validation (this PR)

   Keep spans private but let consumers hook into the resolution process via a Validator trait with callbacks.

   • Pros: No breaking changes; opt-in complexity; zero cost for users who don't need spans
   • Cons: Validation must happen during push_* calls, not after-the-fact inspection

This PR implements Strategy 2 because it avoids breaking changes and has zero cost for existing. However, I'm happy to go with one of the other strategies or something totally new that I haven't considered.

Here's an example of how a consumer would use this new features:

use wit_parser::{Error, Resolve, Span, Validator};

struct NoEmptyRecords;

impl Validator for NoEmptyRecords {
    fn validate_type(&mut self, resolve: &Resolve, id: TypeId, span: Span) -> Result<(), Error> {
        let ty = &resolve.types[id];
        if let TypeDefKind::Record(r) = &ty.kind {
            if r.fields.is_empty() {
                return Err(Error::new(span, "empty records are not allowed"));
            }
        }
        Ok(())
    }
}

let mut resolve = Resolve::default();
let mut validator = NoEmptyRecords;
resolve.push_source_with_validator("test.wit", source, &mut validator)?;

[fitzgen]

Redirecting review because I'm not familiar with wit-parser