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_sourcefile.ts

@@ -44,7 +44,9 @@ function getExtension(fileName: string, mediaType: MediaType): ts.Extension {
       return ts.Extension.Js;
     case MediaType.Unknown:
     default:
-      throw TypeError("Cannot resolve extension.");
+      throw TypeError(
+        `Cannot resolve extension for "${fileName}" with mediaType "${MediaType[mediaType]}".`
+      );
   }
 }