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

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