Addresses concerns brought up in #695
This will make cross-platform implementations more compatible by removing some level of platform option from the mapping process. It will also make it more practical to expose the discussed "profiles" concept by making it easier to have inter-compatible sets of profiles.
Also took the opportunity to normalized on the verbiage "thumbstick" instead of joystick, since that appears to be the official verbiage used by all the device vendors.
该提问来源于开源项目：immersive-web/webxr
