Add UPC, SKU, and manufacturer index builder

.gitignore

@@ -0,0 +1,19 @@
+# Generated exports
+/exports/*
+!/exports/.gitkeep
+
+# Generated indexes
+/indexes/*
+!/indexes/.gitkeep
+!/indexes/upc-index.json
+!/indexes/sku-index.json
+!/indexes/manufacturer-index.json
+
+# Local and temporary files
+*.tmp
+*.bak
+*.log
+.DS_Store
+Thumbs.db
+.vscode/
+.idea/