Add future.v7_normalizeFormMethod flag

[brophdawg11]

Add a future flag to normalize/align useNavigation().formMethod and useFetcher().formMethod with Remix.

We want our API to behave like fetch() - it can accept lowercase or uppercase, but exposes the method back to you as a normalized uppercase method:

new Request("/", { method: "post" }).method === "POST" // true
new Request("/", { method: "POST" }).method === "POST" // true

So this means that both <Form method="post"> and <Form method="POST"> should result in useNavigation().formMethod === "POST".

We inadvertently shipped lowercase useNavigation/useFetcher values in 6.4, and only noticed the inconsistency when we went to deprecate useTransition in Remix which used uppercase. Therefore, we're putting this behind a future flag tro avoid breaking apps, and in Remix v2/React Router v7 we'll move to uppercase as the default.

[changeset-bot]

🦋 Changeset detected

Latest commit: ae419d1

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 5 packages

Name	Type
react-router	Minor
react-router-dom	Minor
@remix-run/router	Minor
react-router-dom-v5-compat	Minor
react-router-native	Minor

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