Add delete items alter hook

Problem/Motivation

I needed to alter deleted item IDs to add additional IDs for records which had been 'chunked' into multiple items on the index to avoid the 10kb size limit - e.g., when item `entity:node/1:en` was deleted, I needed to also delete `entity:node/1:en:chunk:1`. AFAICT there is no search_api event for item deletion?

Proposed resolution

To add an alter hook `hook_search_api_algolia_delete_items_alter()`. Patch attached.