Improve support for diagnostics from runtime compiler APIs (#3911)

- Exports diagnostic items from `diagnostics.ts` which are missing at
  runtime.
- Returns an array of diagnostics, instead of an object with a property
  of `items`.  This is because of the way Rust deals with certain
  structures, and shouldn't be exposed in the APIs.

cli/js/compiler.ts

@@ -255,7 +255,9 @@ async function tsCompilerOnMessage({
 
       assert(emitResult.emitSkipped === false, "Unexpected skip of the emit.");
       const result = [
-        diagnostics.length ? fromTypeScriptDiagnostic(diagnostics) : undefined,
+        diagnostics.length
+          ? fromTypeScriptDiagnostic(diagnostics).items
+          : undefined,
         bundle ? state.emitBundle : state.emitMap
       ];
       globalThis.postMessage(result);