Parses the render options that would be used to
submit the given form, but does not render.
Example
Given a form element:
<form action="/foo" method="post" up-target=".content">
...
</form>
We can parse the link's render options like this:
let form = document.querySelector('form')
let options = up.form.submitOptions(form)
// => { url: '/foo', method: 'POST', target: '.content', ... }
