config: add url.style=none|single|double|curly|dotted|dashed

This option allows you to configure which underline style to use with
URLs.

The default is dotted. Since before this patch, URL underlines were
always plain underlines, this means the default URL underline style
has changed, from single to dotted.

Closes #2302

terminal.h

@@ -105,15 +105,6 @@ struct uri_range_data {
     char *uri;
 };
 
-enum underline_style {
-    UNDERLINE_NONE,
-    UNDERLINE_SINGLE,  /* Legacy underline */
-    UNDERLINE_DOUBLE,
-    UNDERLINE_CURLY,
-    UNDERLINE_DOTTED,
-    UNDERLINE_DASHED,
-};
-
 struct underline_range_data {
     enum underline_style style;
     enum color_source color_src;