feat: programmatic form submissions

[Rich-Harris]

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>

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Latest commit: 60855ef

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Preview: https://svelte-dev-git-preview-kit-15657-svelte.vercel.app/