Wrap drupal_get_js() and drupal_get_css() in classes to remove process layer

Problem/Motivation

We are trying to remove the process layer but in doing so 'getting' all the CSS and JS in the first/main preprocess will build the JS/CSS into a string too early and later preprocesses will need to call drupal_get_css() or drupal_get_js() again to re-populate the variable for the template.

Proposed resolution

This is a stop gap solution that can be improved on with the twig level call to a renderable object.
See the patch a starting patch.

Remaining tasks

Apply this with every other process patch in meta. This patch only does template_process_html() and template_process_maintenance_page(()

API changes

New classes that wrap
drupal_get_js()
drupal_get_css()

Related Issues

#1843650: [meta] Remove the process layer