Reimplement and simplify Hugo's template system

See #13541 for details.

Fixes #13545
Fixes #13515
Closes #7964
Closes #13365
Closes #12988
Closes #4891

common/maps/ordered.go

@@ -14,8 +14,9 @@
 package maps
 
 import (
-	"github.com/gohugoio/hugo/common/hashing"
 	"slices"
+
+	"github.com/gohugoio/hugo/common/hashing"
 )
 
 // Ordered is a map that can be iterated in the order of insertion.
@@ -57,6 +58,15 @@ func (m *Ordered[K, T]) Get(key K) (T, bool) {
 	return value, found
 }
 
+// Has returns whether the given key exists in the map.
+func (m *Ordered[K, T]) Has(key K) bool {
+	if m == nil {
+		return false
+	}
+	_, found := m.values[key]
+	return found
+}
+
 // Delete deletes the value for the given key.
 func (m *Ordered[K, T]) Delete(key K) {
 	if m == nil {