Remove "onBeforeRender" event and add "errors" option to preserveScroll and preserveState

[reinink]

This PR reverts a previous change (#410) that added a new onBeforeRender() event, used by the Vue 2 and Vue 3 form helpers to set form state earlier in the visit lifecycle. As it turns out, this creates other issues, such as the page props being out of date when form.processing returns back to false.

The entire motivation with adding onBeforeRender() was to simplify this:

preserveScroll: (page) => Object.keys(page.props.errors).length > 0,
preserveState: (page) => Object.keys(page.props.errors).length > 0,

To this:

preserveScroll: () => this.form.hasErrors,
preserveState: () => this.form.hasErrors,

Instead of solving the problem that way, I've introduced a new option to preserveScroll and preserveState specifically for preserving the scroll/state when there are errors:

preserveScroll: 'errors',
preserveState: 'errors',

Overall I think this is a much better solution. 👍