Make spellcheck available as an index processor

Problem/Motivation

Currently the module adds spell checking to the search query directly from the view area plugin this makes it impossible to leverage spell checking on the index outside of views. For my use case I'd like to use spellcheck with a decoupled search from search_api_decoupled → but can't trigger spellcheck/add the suggestions to the api response because views isn't being used.

Steps to reproduce

Proposed resolution

To make the spellcheck functionality available outside of views it'd make sense to pull the query alter functionality into a search api prequery processor. This should be possible without a breaking change since the current views behavior can be maintained and thought of as an "override" while the new the processor would be the "default" for the index.

Remaining tasks

User interface changes

API changes

Data model changes