Extend preloadable types

[Kingdutch]

Follow-up to #86

Extend the types of things that we can preload. MDN contains a list of valid values for as that we probably want to try out with e.g. Vite to see how it exposes the data to ensure we can get it into our manifest and preload it with the right time. We may decide that we want to replace the empty arrays (easy to use) with nullable objects (smaller to transfer) if we have many different types in our manifest.