Position nodes absolutely when outroing them, if animations are involved

[Rich-Harris]

Best shown with a GIF:

There's a little bit of magic here, which you opt in to when you use the animate directive. There could be some situations where this approach results in glitchiness, though I can't think of any off the top of my head. (Maybe if the node already has transforms involving scaling/rotation etc?)

This is a little tricky to test with JSDOM... not sure if there's a great way around that 😬

TODO:

• Pass in the destroy function to updateKeyedEach from EachBlock, so that apps that aren't using animate don't have to bear the extra weight