Support for Search API Spellcheck

Update the issue summary, including steps to test.

I did a quick rebase in the form of a patch for alpha2.

Sorry I can't dedicate more effort to this since I'm only test-driving elasticsearch

@rodrigoaguilera No need for patches on 8.0.x branch. One can just type /rebase on Gitlab to rebase the branch.

The tests pass now, seems like "_PHPUNIT_CONCURRENT" was causing issues, increasing the memory resources for Elasticsearch partially helped on the issue.

One question left for @mparker17, otherwise this is good to go.

@sokru, Thanks for flagging, and thank you VERY much for your efforts to fix this. I think PHPStan might have suggested that change, but I think that check should still be there, so I put it back. :)

Going to mark this as RTBC based on @sokru's comment in #8

Thanks, committed.

Automatically closed - issue fixed for 2 weeks with no activity.