Overview

This PR flips the tuple order of useTransition from:

const [startTransition, isPending] = useTransition();

to

const [isPending, startTransition] = useTransition();

Motivation

The new order, with isPending first nicely mirrors the tuple order of useState and useReducer, where the state value is first and the setter is second. This change makes it easier to remember the correct order, by not needing to remember a difference.

We didn't start the order this way from the beginning because for use cases where the pending value is unused, users would need to use the _ pattern often to denote that the pending value is unused:

const [_, startTransition] = useTransition();

However, since then, we've added React.startTranstion() to work outside of hooks, and this can be used when a pending value is not need. So there's no downside to having the same order as other hooks. And as an added benefit, using the hook version pressures users to do something with the isPending value (like show a spinner on a button), which is considered a best practice.

Closes #17276.