Add Goldmark as the new default markdown handler

This commit adds the fast and CommonMark compliant Goldmark as the new default markdown handler in Hugo.

If you want to continue using BlackFriday as the default for md/markdown extensions, you can use this configuration:

```toml
[markup]
defaultMarkdownHandler="blackfriday"
```

Fixes #5963
Fixes #1778
Fixes #6355

docs/config/_default/config.toml

@@ -14,17 +14,11 @@ pluralizeListTitles = false
 # We do redirects via Netlify's _redirects file, generated by Hugo (see "outputs" below).
 disableAliases = true
 
-# Highlighting config (Pygments)
-# It is (currently) not in use, but you can do ```go in a content file if you want to.
-pygmentsCodeFences = true
-
-pygmentsOptions = ""
-# Use the Chroma stylesheet
-pygmentsUseClasses = true
-pygmentsUseClassic = false
-
-# See https://help.farbox.com/pygments.html
-pygmentsStyle = "trac"
+[markup]
+[markup.highlight]
+style = "trac"
+lineNumbersInTable = true
+noClasses = false
 
 [outputs]
 home = [ "HTML", "RSS", "REDIR", "HEADERS" ]