guisp support on generate

[PBird]

Hello, i was making theme, and i realize generater doesn't have guisp support. Can you add guisp color support to change undercurl color.

example:
CocErrorHighlight: "red - c[red]" -> hi CocErrorHighlight gui=undercurl guisp=red

Fixed:
there is support guisp
CocErrorHighlight: "- - c error_red"