Font::from_guifont does not handle list

[wezm]

guifont can be set to a list of fonts but gnvim only supports parsing a single font

https://github.com/vhakulinen/gnvim/blob/6ba47373545c6e9be3677a3ae695415809cf2fdf/src/ui/font.rs#L33

					*'guifont'* *'gfn'*
						   *E235* *E596*
'guifont' 'gfn'		string	(default "")
			global
	This is a list of fonts which will be used for the GUI version of Vim.
	In its simplest form the value is just one font name.  When
	the font cannot be found you will get an error message.  **To try other
	font names a list can be specified, font names separated with commas.**
	The first valid font is used.