tpl/transform: Add transform.Unmarshal func

Fixes #5428

tpl/transform/transform.go

@@ -19,6 +19,8 @@ import (
 	"html"
 	"html/template"
 
+	"github.com/gohugoio/hugo/cache/namedmemcache"
+
 	"github.com/gohugoio/hugo/deps"
 	"github.com/gohugoio/hugo/helpers"
 	"github.com/spf13/cast"
@@ -26,14 +28,22 @@ import (
 
 // New returns a new instance of the transform-namespaced template functions.
 func New(deps *deps.Deps) *Namespace {
+	cache := namedmemcache.New()
+	deps.BuildStartListeners.Add(
+		func() {
+			cache.Clear()
+		})
+
 	return &Namespace{
-		deps: deps,
+		cache: cache,
+		deps:  deps,
 	}
 }
 
 // Namespace provides template functions for the "transform" namespace.
 type Namespace struct {
-	deps *deps.Deps
+	cache *namedmemcache.Cache
+	deps  *deps.Deps
 }
 
 // Emojify returns a copy of s with all emoji codes replaced with actual emojis.