chore(firestore-stripe-invoices): cleanup of source files, added prepare build step (#357)

* chore: cleanup of source files, added prepare build step

* removed lib folder, cleanup dependency added

* added clean fn to build-watch task

* removed obsolete lib files

Author: dackers86

firestore-stripe-invoices/functions/.gitignore

@@ -1,2 +1,4 @@
 node_modules/
-firebase.json
+firebase.json
+
+lib