Defines a font family and its source for use in brand typography. When `source` is `"system"`, a `dir_source` path to the local font files must be provided.
Arguments
- family
A character string with the font family name (e.g. `"Helvetica Neue"`).
- source
A character string specifying the font source. Must be one of `"system"` (local files) or `"google"` (Google Fonts).
- dir_source
A character string with the path to the directory containing local font files. Required when `source` is `"system"`, ignored otherwise.