Text-family names for the character-cell plotting backend.
Preferred public API for the backend that renders plots as text. The
ASCII charset is its current default; future modes (Unicode, braille,
ANSI color) share this family. fortplot_ascii stays the concrete
implementation and a working compatibility API, so both
use fortplot_text_backend and use fortplot_ascii compile. The
name fortplot_text is already the backend-agnostic font/layout module,
so the backend family uses fortplot_text_backend.