breaking: `invalid` now must be imported from `@sveltejs/kit` (#14768)

breaking: `invalid` now must be imported from `@sveltejs/kit` (#14768)

* breaking: `invalid` now must be imported from `@sveltejs/kit`

TypeScript kinda forced our hand here - due to limitations of control flow analysis it does not detect the `never` return type for anything else than functions that are used directly (i.e. passing a function as a parameter doesn't work unless you explicitly type it); see https://github.com/microsoft/TypeScript/issues/36753 for more info.

This therefore changes `invalid` to be a function that you import just like `redirect` or `error`. A nice benefit of this is that you'll no longer have to use the second parameter passed to remote form functions to construct the list of issues in case you want to create an issue for the whole form and not just a specific field.

Closes #14745

* docs

* fix test

* Apply suggestions from code review

* Update packages/kit/src/runtime/app/server/remote/form.js

* Update documentation/docs/20-core-concepts/60-remote-functions.md

* prettier

* regenerate

* fix

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Co-authored-by: Rich Harris <richard.a.harris@gmail.com>
Co-authored-by: Rich Harris <rich.harris@vercel.com>