This commit will introduce the Hyku::Indexers::FileSetIndexer to add
indexing logic for born digital PDFs when using PDF.js.  We also change
the works' indexing field to match the file sets' indexing field
(all_text_tsimv).  We also "valyrized" the logic in the HykuIndexing
module to accomplish this.

Ref:
- scientist-softserv/adventist_knapsack#769

This commit will add logic to add the ability to see search snippets
with PDFs that were split through IIIF Print.

This commit will add a simple test for the FileSetIndexer logic to check
that the text extraction from a born digital pdf works as expected.

This commit will rename the #full_text method to #extract_full_text
because it was causing weird issues with super.