tpl: Add TemplateFuncsNamespaceRegistry

As a first step to remove the hard ties between `tplimpl` and the different namespace packages.

The `lang` package is used as the first example use case.

See #3042

tpl/internal/templatefuncsRegistry.go

@@ -0,0 +1,47 @@
+// Copyright 2017-present The Hugo Authors. All rights reserved.
+//
+// Portions Copyright The Go Authors.
+
+// 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 internal
+
+import (
+	"github.com/spf13/hugo/deps"
+)
+
+var TemplateFuncsNamespaceRegistry []func(d *deps.Deps) *TemplateFuncsNamespace
+
+func AddTemplateFuncsNamespace(ns func(d *deps.Deps) *TemplateFuncsNamespace) {
+	TemplateFuncsNamespaceRegistry = append(TemplateFuncsNamespaceRegistry, ns)
+}
+
+type TemplateFuncsNamespace struct {
+	// The namespace name, "strings", "lang", etc.
+	Name string
+
+	// This is the method receiver.
+	Context interface{}
+
+	// Any template funcs aliases. This is mainly motivated by keeping
+	// backwards compability, but some new template funcs may also make
+	// sense to give short and snappy aliases.
+	// Note that these aliases are global and will be merged, so the last
+	// key will win.
+	Aliases map[string]interface{}
+
+	// A slice of input/expected examples.
+	// We keep it a the namespace level for now, but may find a way to keep track
+	// of the single template func, for documentation purposes.
+	// Some of these, hopefully just a few, may depend on some test data to run.
+	Examples [][2]string
+}