Make Page an interface

The main motivation of this commit is to add a `page.Page` interface to replace the very file-oriented `hugolib.Page` struct.
This is all a preparation step for issue  #5074, "pages from other data sources".

But this also fixes a set of annoying limitations, especially related to custom output formats, and shortcodes.

Most notable changes:

* The inner content of shortcodes using the `{{%` as the outer-most delimiter will now be sent to the content renderer, e.g. Blackfriday.
  This means that any markdown will partake in the global ToC and footnote context etc.
* The Custom Output formats are now "fully virtualized". This removes many of the current limitations.
* The taxonomy list type now has a reference to the `Page` object.
  This improves the taxonomy template `.Title` situation and make common template constructs much simpler.

See #5074
Fixes #5763
Fixes #5758
Fixes #5090
Fixes #5204
Fixes #4695
Fixes #5607
Fixes #5707
Fixes #5719
Fixes #3113
Fixes #5706
Fixes #5767
Fixes #5723
Fixes #5769
Fixes #5770
Fixes #5771
Fixes #5759
Fixes #5776
Fixes #5777
Fixes #5778

tpl/tplimpl/template_test.go

@@ -1,66 +0,0 @@
-// Copyright 2017-present 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
-
-import (
-	"testing"
-
-	"github.com/gohugoio/hugo/deps"
-	"github.com/gohugoio/hugo/hugofs"
-	"github.com/gohugoio/hugo/tpl"
-	"github.com/stretchr/testify/require"
-)
-
-type handler interface {
-	addTemplate(name, tpl string) error
-}
-
-// #3876
-func TestHTMLEscape(t *testing.T) {
-	assert := require.New(t)
-
-	data := map[string]string{
-		"html":  "<h1>Hi!</h1>",
-		"other": "<h1>Hi!</h1>",
-	}
-	v := newTestConfig()
-	fs := hugofs.NewMem(v)
-
-	//afero.WriteFile(fs.Source, filepath.Join(workingDir, "README.txt"), []byte("Hugo Rocks!"), 0755)
-
-	depsCfg := newDepsConfig(v)
-	depsCfg.Fs = fs
-	d, err := deps.New(depsCfg)
-	assert.NoError(err)
-
-	templ := `{{ "<h1>Hi!</h1>" | safeHTML }}`
-
-	provider := DefaultTemplateProvider
-	provider.Update(d)
-
-	h := d.Tmpl.(handler)
-
-	assert.NoError(h.addTemplate("shortcodes/myShort.html", templ))
-
-	tt, _ := d.Tmpl.Lookup("shortcodes/myShort.html")
-	s, err := tt.(tpl.TemplateExecutor).ExecuteToString(data)
-	assert.NoError(err)
-	assert.Contains(s, "<h1>Hi!</h1>")
-
-	tt, _ = d.Tmpl.Lookup("shortcodes/myShort")
-	s, err = tt.(tpl.TemplateExecutor).ExecuteToString(data)
-	assert.NoError(err)
-	assert.Contains(s, "<h1>Hi!</h1>")
-
-}