Add author_name virtual field for use by oik-blocks/fields

[bobbingwide]

Requirement

• Support display of the author_name for a post using a block.

Solution

• Add a new virtual field author_name
• Implement the callback function bw_fields_get_author_name() using get_the_author()

Note: It's also possible to use the core/post-author block, with the settings adjusted to not display the Avatar or Bio
and the "Byline" set as required for the label, if needed.

See bobbingwide/oik-blocks#45 and bobbingwide/sb-field-block#5