useTransition API rewrite

[joshuaellis]

A clear and concise description of what the feature is

Rewrite useTransition (packages/core/src/hooks/useTransition.tsx) around a declarative enter/leave model — presence made explicit, "obvious on first read" — without the render-prop bookkeeping it has today:

// today — you index into a render prop, and it's unclear where `item` comes from
const transitions = useTransition(items, {
  keys: item => item.id,
  from: { opacity: 0 },
  enter: { opacity: 1 },
  leave: { opacity: 0 },
})
return transitions((style, item) => (
  <animated.div style={style}>{item.text}</animated.div>
))

Would also subsume #2136 (per-property configs should fall out of the new model rather than be bolted on).

Why should this feature be included?

useTransition is the library's most-reported source of confusion — the same friction recurs across years:

• RFC / useTransition 🤯 and possible improvements #665 (2019) — the API was called "a little weird": users "see item down in the view section and don't know where it comes from."
• How do I use the new useTransition API to trigger a  #1281 (2021) — confusion over where state fits once the API takes items.
• How to use Page transitions in next.js #1711 (discussion) — "how do I make it wait so the leave transition can have an effect?" — exit timing isn't discoverable.

Related issues

• [feat]: Animate Presence component addition #1955 — <Presence> component; the enter/exit primitive a cleaner useTransition unlocks.
• Animations and persistence of components on route changes #1628 — persisting/animating components across route changes; a real-world presence use case (its shared-layout half maps to layoutId, tracked separately).

Scope / non-goals

• The exact new API shape is what this issue is to settle.
• It must keep react-spring's model: you own the springs and the new API orchestrates them. Presence must not become props on animated.* that create/own springs internally — that inverts the model (see the reasoning on [feat]: Create an animation abstraction for spring and gesture #1826).