Custom dynamic assets

[NiklasEi]

Support loading and using custom types as dynamic assets.

E.g. it should be possible (with minimal setup), to automatically load files like this:

({
    "bakery": Building (
        position: [100., 0., 10.],
        image: "images/bakery.png"
    ),
    "baker": Npc (
        spawn: [100., 0., 10.],
        image: "images/baker.png",
    ),
})

This should not be bound to a certain file format.