Restructure frontend code to allow for extensions (#4721)

# Description of Changes
Move frontend code into `core` folder and add infrastructure for
`proprietary` folder to include premium, non-OSS features

frontend/src/core/tests/utils/testFileHelpers.ts

@@ -0,0 +1,28 @@
+/**
+ * Test utilities for creating StirlingFile objects in tests
+ */
+
+import { StirlingFile, createStirlingFile } from '@app/types/fileContext';
+
+/**
+ * Create a StirlingFile object for testing purposes
+ */
+export function createTestStirlingFile(
+  name: string,
+  content: string = 'test content',
+  type: string = 'application/pdf'
+): StirlingFile {
+  const file = new File([content], name, { type });
+  return createStirlingFile(file);
+}
+
+/**
+ * Create multiple StirlingFile objects for testing
+ */
+export function createTestFilesWithId(
+  files: Array<{ name: string; content?: string; type?: string }>
+): StirlingFile[] {
+  return files.map(({ name, content = 'test content', type = 'application/pdf' }) =>
+    createTestStirlingFile(name, content, type)
+  );
+}