Split font shorthand property into a named list of verbose "font-*" properties

This is not perfect/finished but it is a step in the right direction.

split_font_shorthand(font_text)

Arguments

font_text	the text of the "font" declaration e.g. "italic 1.2em "Fira Sans", serif;"

list of font-related property/value declarations e.g. list('font-family'='serif', font-size='1.2em', ...)

For example: this function does not split the "font-variant" shorthand