[v9] Make useAsset return a thenable

[alexandernanberg]

Currently useAsset suspends by itself. While this is fine for most cases, but there are cases where you want more control. My proposal is that useAsset instead return a Thenable which consumers unwrap with use().

E.g.

const assetPromise = useAsset(GLTFLoader, '/path/to/model.glb')
const asset = use(assetPromise)

or a one-liner

const asset = use(useAsset(GLTFLoader, '/path/to/model.glb'))

Note that I we do this, useAsset doesn't really need to be a hook any more. So we could even do something like

const asset = use(loadAsset(GLTFLoader, '/path/to/model.glb'))