Helper function. Checks if a given family value is available, and if not returns the default font family ("sans" or user provided)
get_usable_family(family, silent = FALSE, default_family = "sans")
a character of a usable font family
the font family to check as a character
logical. If TRUE doesn't raise a warning if the font family is unavailable
defaults to "sans", but can be set to another fallback family.