Sensor API: expose separate user acceleration and gravity sensor types

[slime73]

Right now total device acceleration (user acceleration plus acceleration from gravity) is exposed as a sensor type. This is useful on its own, but some devices/APIs also expose individual user acceleration and gravity sensors as well (I know iOS does for sure). They're more expensive because I think they need gyroscope data internally to compute the split, but it's also pretty useful to have access to those when they're supported. I don't think they should replace the total acceleration sensor type.

For reference here's some iOS APIs for them: https://developer.apple.com/documentation/coremotion/cmdevicemotion?language=objc

[slouken]

I didn't expose these because they're not supported on any other system, so if you want cross-platform support you'll have to roll your own. In theory we could roll our own and implement this for other platforms as well, but that is a larger project for someone.

[slime73]

Android seems to support them: https://developer.android.com/guide/topics/sensors/sensors_overview

Apple's MFi joystick API (iOS and macOS) does as well.