refactor(localize): hide merging diagnostics messages behind a method (#36792)

This commit introduces `Diagnostics.merge(other)` which will take the messages from
`other` and append them to the messages of `this`.

The translation loader is updated to use this new method.

PR Close #36792

packages/localize/src/tools/src/diagnostics.ts

@@ -31,6 +31,9 @@ export class Diagnostics {
   error(message: string) {
     this.messages.push({type: 'error', message});
   }
+  merge(other: Diagnostics) {
+    this.messages.push(...other.messages);
+  }
   formatDiagnostics(message: string): string {
     const errors = this.messages!.filter(d => d.type === 'error').map(d => ' - ' + d.message);
     const warnings = this.messages!.filter(d => d.type === 'warning').map(d => ' - ' + d.message);

packages/localize/src/tools/src/translate/translation_files/translation_loader.ts

@@ -64,7 +64,7 @@ export class TranslationLoader {
 
         // If we were passed a diagnostics object then copy the messages over to it.
         if (this.diagnostics) {
-          this.diagnostics.messages.push(...diagnostics.messages);
+          this.diagnostics.merge(diagnostics);
         }
 
         return {locale, translations, diagnostics};