Tweak highlighting when trait is available for different type

When printing

```
  = help: the trait `chumsky::private::ParserSealed<'_, &'a str, ((), ()), chumsky::extra::Full<EmptyErr, (), ()>>` is implemented for `Then<Ignored<chumsky::combinator::Filter<chumsky::primitive::Any<&str, chumsky::extra::Full<EmptyErr, (), ()>>, {closure@src/main.rs:9:17: 9:27}>, char>, chumsky::combinator::Map<impl CSTParser<'a, O>, O, {closure@src/main.rs:11:24: 11:27}>, (), (), chumsky::extra::Full<EmptyErr, (), ()>>`
  = help: for that trait implementation, expected `((), ())`, found `()`
```

Highlight only the `expected` and `found` types, instead of the full type in the first `help`.

Author: estebank

compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs

@@ -1818,23 +1818,32 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                     if impl_trait_ref.references_error() {
                         return false;
                     }
+                    let self_ty = impl_trait_ref.self_ty().to_string();
                     err.highlighted_help(vec![
                         StringPart::normal(format!(
                             "the trait `{}` ",
                             impl_trait_ref.print_trait_sugared()
                         )),
                         StringPart::highlighted("is"),
                         StringPart::normal(" implemented for `"),
-                        StringPart::highlighted(impl_trait_ref.self_ty().to_string()),
+                        if let [TypeError::Sorts(_)] = &terrs[..] {
+                            StringPart::normal(self_ty)
+                        } else {
+                            StringPart::highlighted(self_ty)
+                        },
                         StringPart::normal("`"),
                     ]);
 
                     if let [TypeError::Sorts(exp_found)] = &terrs[..] {
                         let exp_found = self.resolve_vars_if_possible(*exp_found);
-                        err.help(format!(
-                            "for that trait implementation, expected `{}`, found `{}`",
-                            exp_found.expected, exp_found.found
-                        ));
+                        err.highlighted_help(vec![
+                            StringPart::normal("for that trait implementation, "),
+                            StringPart::normal("expected `"),
+                            StringPart::highlighted(exp_found.expected.to_string()),
+                            StringPart::normal("`, found `"),
+                            StringPart::highlighted(exp_found.found.to_string()),
+                            StringPart::normal("`"),
+                        ]);
                     }
 
                     true