[css-animations-2][web-animation-2] Rename exit range to active range

[vmpstr]

In scroll triggered animations, we have a notion of an exit range. This acts as a range in which the animation stays active once it has been triggered.

To borrow from pseudo classes, specifically :active, maybe this range is better named as "active range".

Here's a demo that @flackr put together for the analogy to the :hover state:
https://jsbin.com/yusecom/edit?html,css,output

TLDR: Let's rename the exit range and all corresponding properties to active range

@bramus @DavMila @ydaniv

[flackr]

At the same time if we change this we should probably change the spec terminology from "enter" to "activation" or something similar, giving us the activation range and the active range, which makes sense to me.