Use Chroma as new default syntax highlighter

If you want to use Pygments, set `pygmentsUseClassic=true` in your site config.

Fixes #3888

helpers/testhelpers_test.go

@@ -34,5 +34,9 @@ func newTestCfg(fs *hugofs.Fs) *viper.Viper {
 
 func newTestContentSpec() *ContentSpec {
 	v := viper.New()
-	return NewContentSpec(v)
+	spec, err := NewContentSpec(v)
+	if err != nil {
+		panic(err)
+	}
+	return spec
 }