Add API to interpolate between two free camera positions

Currently there's no easy way to interpolate smoothly between two free-form camera positions. Existing examples interpolate lng/lat/latitude separately (which will have issues on bigger distances due non-linear latitude to y relation) and/or use lookAt method to sidestep interpolating rotation, which is not an applicable strategy in many cases.

In an example I played with for smooth animation along path, I had to interpolate manually using glMatrix spherical quaternion interpolation:

// interpolate position
camera.position.x = sourceCamera.position.x + (targetCamera.position.x - sourceCamera.position.x) * t;
camera.position.y = sourceCamera.position.y + (targetCamera.position.y - sourceCamera.position.y) * t;
camera.position.z = sourceCamera.position.z + (targetCamera.position.z - sourceCamera.position.z) * t;

// interpolate orientation
glMatrix.quat.slerp(camera.orientation, sourceCamera.orientation, targetCamera.orientation, t);

We should have an easy API for this, e.g.:

const camera = mapboxgl.FreeCameraOptions.interpolate(sourceCamera, targetCamera, t);