Use structs for JSON serialization

[ntBre]

Summary

See #19133 (comment) for recent discussion. This PR moves to using structs for the types in our JSON output format instead of the json! macro.

I didn't rename any of the message references because that should be handled when rebasing #19133 onto this.

My plan for handling the preview behavior with the new diagnostics is to use a wrapper enum. Something like:

#[derive(Serialize)]
#[serde(untagged)]
pub(crate) enum JsonDiagnostic<'a> {
    Old(OldJsonDiagnostic<'a>),
}

#[derive(Serialize)]
pub(crate) struct OldJsonDiagnostic<'a> {
    // ...
}

Initially I thought I could use a &dyn Serialize for the affected fields, but I see that Serialize isn't dyn-compatible in testing this now.

Test Plan

Existing tests. One quirk of the new types is that their fields are in alphabetical order. I guess json! sorts the fields alphabetically? The tests were failing before I sorted the struct fields.

Other formats

It looks like the rdjson, sarif, and gitlab formats also use json!, so if we decide to merge this, I can do something similar for those before moving them to the new diagnostic format.

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

[MichaReiser]

I'm leaning towards reverting the last commit. The old implementation carefully avoided allocating a new vector for the edits. I also don't think that changing this is necessary for making this more typed. We can just replace the

           let value = json!({
                "content": edit.content().unwrap_or_default(),
                "location": location_to_json(location),
                "end_location": location_to_json(end_location)
            });

and initialize and then serialize said struct instead.

[ntBre]

Makes sense, reverted!