Support for A to Z pagination

[bobbingwide]

To solve some mega data problems with WP-a2z

hookr.io's UI is highly interactive.
WP-a2z needs to introduce a better way of filtering the displayed content.
The solution is expected to use REST/AJAX to obtain the data and React to display it, but it will also require back end logic to automatically create the "letter" taxonomies used by the REST API.
Note: The back end logic may be implemented using a plugin.

[bobbingwide]

The backend logic to create the letter taxonomies may be dependent upon bobbingwide/oik-a2z#1

[bobbingwide]

To reflect updates to genesis-oik for Blocks and Block examples we need to create templates for single-block and single-block_example. Copy and cobble single-shortcodes

[bobbingwide]

A to Z pagination should also work correctly on Taxonomy archive pages.
Currently it displays the wrong letter taxonomy for the block related taxonomies.