Add Markdown diagrams and render hooks for code blocks

You can now create custom hook templates for code blocks, either one for all (`render-codeblock.html`) or for a given code language (e.g. `render-codeblock-go.html`).

We also used this new hook to add support for diagrams in Hugo:

* Goat (Go ASCII Tool) is built-in and enabled by default; just create a fenced code block with the language `goat` and start draw your Ascii diagrams.
* Another popular alternative for diagrams in Markdown, Mermaid (supported by GitHub), can also be implemented with a simple template. See the Hugo documentation for more information.

Updates #7765
Closes #9538
Fixes #9553
Fixes #8520
Fixes #6702
Fixes #9558

tpl/transform/remarshal_test.go

@@ -11,13 +11,14 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-package transform
+package transform_test
 
 import (
 	"testing"
 
-	"github.com/gohugoio/hugo/config"
 	"github.com/gohugoio/hugo/htesting"
+	"github.com/gohugoio/hugo/hugolib"
+	"github.com/gohugoio/hugo/tpl/transform"
 
 	qt "github.com/frankban/quicktest"
 )
@@ -25,13 +26,14 @@ import (
 func TestRemarshal(t *testing.T) {
 	t.Parallel()
 
-	v := config.New()
-	v.Set("contentDir", "content")
-	ns := New(newDeps(v))
+	b := hugolib.NewIntegrationTestBuilder(
+		hugolib.IntegrationTestConfig{T: t},
+	).Build()
+
+	ns := transform.New(b.H.Deps)
 	c := qt.New(t)
 
 	c.Run("Roundtrip variants", func(c *qt.C) {
-
 		tomlExample := `title = 'Test Metadata'
 		
 [[resources]]
@@ -129,7 +131,6 @@ title: Test Metadata
 
 			}
 		}
-
 	})
 
 	c.Run("Comments", func(c *qt.C) {