Add more options for EmitterPosition

[DaemonLoki]

The EmitterPosition option for CAEmitterLayer-based effects is pretty limited right now. It only has 3 options:

1. .top
2. .center
3. .bottom

From an implementation standpoint, it is rather straightforward to expand that to more options, like .topLeading, .bottomTrailing, .centerLeading, and all combinations of them.

The calculation of the position also doesn't require much effort as the values are all given due to the embedding in the GeometryReader.