Request rename MLDevicePreference to MLDeviceType to align with WebNN API #41

The previous landed patch webmachinelearning/webnn#257 of WebNN API renamed MLDevicePreference to MLDeviceType with reason "rename MLDevicePreference to MLDeviceType to make it more clear that it is not merely a hint".

Current WebNN API samples is renaming MLDevicePreference to MLDeviceType by webmachinelearning/webnn-samples#150 to align latest WebNN API Spec, while device selection would not work by running Chrome with enabled enable-experimental-web-platform-features flag,

//  WebNN API code
navigator.ml.createContext({deviceType: 'cpu'})
navigator.ml.createContext({deviceType: 'gpu'})

so we request rename MLDevicePreference to MLDeviceType for Model Loader API, thanks.