Return an AssetByteReader object that can be used to read the contents of the asset.
reader()
An AssetByteReader object.
The AssetByteReader that has a method read(length) that reads length bytes
from the asset and returns ArrayBuffer. It keeps track of the offset in the asset,
and updates it with each read. When the end of the asset is reached, it returns undefined.
let reader = Asset("myasset").reader()
reader.read(6)
ArrayBuffer [ 0, 1, 2, 3, 4, 5 ]