markup/goldmark: Add the Hugo Goldmark Extras "delete" extension

With Goldmark v1.7.1 and earlier, the Goldmark "strikethrough" extension was
triggered by wrapping text within a pair of double-tilde characters. With
Goldmark v1.7.2 and later, to provide full GFM compatibility, the Goldmark
"strikethrough" extension is triggered by wrapping text within a pair of
single- or double-tilde characters.

This change created a conflict with the Hugo Goldmark Extras "subscript"
extension.

When enabling the Hugo Goldmark Extras "subscript" extension, if you
want to render subscript and strikethrough text concurrently, you must:

1. Disable the Goldmark "strikethrough" extension
2. Enable the Hugo Goldmark Extras "delete" extension

Closes #12597

markup/goldmark/goldmark_config/config.go

@@ -50,10 +50,7 @@ var Default = Config{
 			EscapedSpace:             false,
 		},
 		Extras: Extras{
-			Superscript: Superscript{
-				Enable: false,
-			},
-			Subscript: Subscript{
+			Delete: Delete{
 				Enable: false,
 			},
 			Insert: Insert{
@@ -62,6 +59,12 @@ var Default = Config{
 			Mark: Mark{
 				Enable: false,
 			},
+			Subscript: Subscript{
+				Enable: false,
+			},
+			Superscript: Superscript{
+				Enable: false,
+			},
 		},
 		Passthrough: Passthrough{
 			Enable: false,
@@ -168,12 +171,17 @@ type Typographer struct {
 // Extras holds extras configuration.
 // github.com/hugoio/hugo-goldmark-extensions/extras
 type Extras struct {
+	Delete      Delete
 	Insert      Insert
 	Mark        Mark
 	Subscript   Subscript
 	Superscript Superscript
 }
 
+type Delete struct {
+	Enable bool
+}
+
 type Insert struct {
 	Enable bool
 }