Reimplement and simplify Hugo's template system

See #13541 for details.

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

tpl/tplimpl/template_info.go

@@ -0,0 +1,46 @@
+// Copyright 2025 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package tplimpl
+
+// Increments on breaking changes.
+const TemplateVersion = 2
+
+// ParseInfo holds information about a parsed ntemplate.
+type ParseInfo struct {
+	// Set for shortcode templates with any {{ .Inner }}
+	IsInner bool
+
+	// Set for partials with a return statement.
+	HasReturn bool
+
+	// Config extracted from template.
+	Config ParseConfig
+}
+
+func (info ParseInfo) IsZero() bool {
+	return info.Config.Version == 0
+}
+
+// ParseConfig holds configuration extracted from the template.
+type ParseConfig struct {
+	Version int
+}
+
+var defaultParseConfig = ParseConfig{
+	Version: TemplateVersion,
+}
+
+var defaultParseInfo = ParseInfo{
+	Config: defaultParseConfig,
+}