Add ~/Library/Fonts to default fontconfig directories

[telotortium (Robert Irelan)]

~/Library/Fonts is [documented (​https://support.apple.com/guide/font-book/change-font-book-preferences-fntbk1004/mac)] as the official location for user-installed fonts. Since we include the [system-installed fonts (​https://github.com/macports/macports-ports/blob/d275a24edbefe7f361f2aa6eb8f59648092058c0/graphics/fontconfig/Portfile#L104-L106)] in the font directory list, as well as ~/.fonts, it stands to reason that we should include ~/Library/Fonts too.