This implements #14082 (comment):

• Remote form instances have a programmatic submit() method (await myform.submit())
• This runs preflight checks, then submits the contents of the form (using the default submitter). It does not run logic inside an enhance callback
• It returns a Promise with an updates method that can be used for optimistic updates
• Enhance callbacks now receive a copy of the instance, rather than a { form, data, submit } object. (It's a copy because we don't want people to just call instance.submit() inside the enhance callback, since that would result in preflight validation running twice)

To facilitate this I made a small change to the implementation — instead of enhance(...) returning a fresh object, it just stashes away the callback for later. This is simpler and more consistent with how preflight works — it means you can do this, for example, without having to stash away a reference in order to spread it:

<script>
  import { myform } from './stuff.remote';

  myform.preflight(schema).enhance(async (f) => {
    // ...
  });
</script>

<form {...myform}>...</form>

Please don't delete this checklist! Before submitting the PR, please make sure you do the following:

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests

• Run the tests with pnpm test and lint the project with pnpm lint and pnpm check

Changesets

• If your PR makes a change that should be noted in one or more packages' changelogs, generate a changeset by running pnpm changeset and following the prompts. Changesets that add features should be minor and those that fix bugs should be patch. Please prefix changeset messages with feat:, fix:, or chore:.

Edits

• Please ensure that 'Allow edits from maintainers' is checked. PRs without this option may be closed.