Allow current color as valid css color

In SVG images `curentColor` is allowed as a way to explicitly inherit the current color. When setting `currentColor` as fill color on a shape together with a styling class, the class will now be used instead of the theme, which was not possible otherwise.

d2graph/color_helper.go

@@ -4,6 +4,7 @@ import "regexp"
 
 // namedColors is a list of valid CSS colors
 var namedColors = []string{
+	"currentcolor",
 	"transparent",
 	"aliceblue",
 	"antiquewhite",