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

1 files changed, 2 insertions, 2 deletions

diff --git a/hugolib/testhelpers_test.go b/hugolib/testhelpers_test.go
index 08949a1fd..d861a5e09 100644
--- a/hugolib/testhelpers_test.go
+++ b/hugolib/testhelpers_test.go
@@ -743,7 +743,7 @@ func (th testHelper) assertFileContent(filename string, matches ...string) {
 	content := readDestination(th, th.Fs, filename)
 	for _, match := range matches {
 		match = th.replaceDefaultContentLanguageValue(match)
-		th.Assert(strings.Contains(content, match), qt.Equals, true, qt.Commentf(content))
+		th.Assert(strings.Contains(content, match), qt.Equals, true, qt.Commentf(match+" not in: \n"+content))
 	}
 }
 
@@ -755,7 +755,7 @@ func (th testHelper) assertFileContentRegexp(filename string, matches ...string)
 		r := regexp.MustCompile(match)
 		matches := r.MatchString(content)
 		if !matches {
-			fmt.Println(content)
+			fmt.Println(match+":\n", content)
 		}
 		th.Assert(matches, qt.Equals, true)
 	}