Add a Views area plugin

Problem/Motivation

Facets 3 and facets_exposed_filters inspired me to try to implement search_api_sorts as a Views plugin. This will give a little more flexibility in how the sorts can be rendered. Personally, I wanted to place the sorts in a view header.

Proposed resolution

• Extract the code for rendering the sorts item list out from SearchApiSortsBlock into a new service.
• Add a new Views area plugin that renders the item list.

Remaining tasks

I've written the changes with an up-to-date Core API. That included the most recent attributes, including an OO hook. This has implications for backward compatibility. At a minimum this would force the supported version to have to be increased to 10.2 (I think). But even to do that I'd have to include a LegacyHook in the .module file. That's fine, but I didn't do it pending a maintainer decision. Without the LegacyHook the minimum Core version will have to increase to 11.1. Let me know what you'd like to do.

User interface changes

API changes

Sort item list rendering is moved to a new service.

Data model changes