Remove hook_forms and implement WebformClientForm

Committed and pushed to 8.x-4.x.

This is very much a first pass, but it's a start:

1. Removes hook_forms and webform_client_form() replaces it with WebformClientForm
2. All process/validate/submit functions in WebformClientForm are still to be converted.
3. Actually converts webform-form.html.twig to twig, before it was just renamed as per #2105547-2: Convert template files to twig files. Conversion based on 2105547.
4. Adds webform.webform to webform.libraries.yml