refactor: Use Cow rather than a Box<dyn Display>

Since we're already behind a method, let's rely on the implementors to
specify the dynamic display and use Cow for common case where the code
is well-known and static, i.e. "TS1234".

crates/codegen/parser/runtime/src/diagnostic.rs

@@ -1,5 +1,5 @@
+use std::borrow::Cow;
 use std::error::Error;
-use std::fmt::Display;
 
 use crate::text_index::TextRange;
 
@@ -13,7 +13,9 @@ pub enum Severity {
 
 pub trait Diagnostic: Error {
     fn range(&self) -> TextRange;
-    fn code(&self) -> Box<dyn Display>;
+    fn code(&self) -> Option<Cow<'_, str>> {
+        None
+    }
     fn severity(&self) -> Severity;
     fn message(&self) -> String;
 }

crates/codegen/parser/runtime/src/napi_interface/diagnostic.rs

@@ -46,6 +46,6 @@ impl Diagnostic {
 
     #[napi]
     pub fn code(&self) -> String {
-        self.0.code().to_string()
+        self.0.code().unwrap_or_default().into_owned()
     }
 }

crates/codegen/parser/runtime/src/parse_error.rs

@@ -59,10 +59,6 @@ impl Diagnostic for ParseError {
         self.text_range.clone()
     }
 
-    fn code(&self) -> Box<dyn std::fmt::Display> {
-        Box::new("ParseError")
-    }
-
     fn severity(&self) -> diagnostic::Severity {
         diagnostic::Severity::Error
     }