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/services/fileStubHelpers.ts

@@ -0,0 +1,34 @@
+import { StirlingFile, StirlingFileStub } from '@app/types/fileContext';
+import { createChildStub, generateProcessedFileMetadata } from '@app/contexts/file/fileActions';
+import { createStirlingFile } from '@app/types/fileContext';
+import { ToolId } from '@app/types/toolId';
+
+/**
+ * Create StirlingFiles and StirlingFileStubs from exported files
+ * Used when saving page editor changes to create version history
+ */
+export async function createStirlingFilesAndStubs(
+  files: File[],
+  parentStub: StirlingFileStub,
+  toolId: ToolId
+): Promise<{ stirlingFiles: StirlingFile[], stubs: StirlingFileStub[] }> {
+  const stirlingFiles: StirlingFile[] = [];
+  const stubs: StirlingFileStub[] = [];
+
+  for (const file of files) {
+    const processedFileMetadata = await generateProcessedFileMetadata(file);
+    const childStub = createChildStub(
+      parentStub,
+      { toolId, timestamp: Date.now() },
+      file,
+      processedFileMetadata?.thumbnailUrl,
+      processedFileMetadata
+    );
+
+    const stirlingFile = createStirlingFile(file, childStub.id);
+    stirlingFiles.push(stirlingFile);
+    stubs.push(childStub);
+  }
+
+  return { stirlingFiles, stubs };
+}