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

magefile.go

@@ -15,6 +15,9 @@ import (
 	"sync"
 	"time"
 
+	"github.com/gohugoio/hugo/codegen"
+	"github.com/gohugoio/hugo/resources/page/page_generate"
+
 	"github.com/magefile/mage/mg"
 	"github.com/magefile/mage/sh"
 )
@@ -64,7 +67,37 @@ func flagEnv() map[string]string {
 }
 
 func Generate() error {
-	return sh.RunWith(flagEnv(), goexe, "generate", path.Join(packageName, "tpl/tplimpl/embedded/generate"))
+	generatorPackages := []string{
+		"tpl/tplimpl/embedded/generate",
+		//"resources/page/generate",
+	}
+
+	for _, pkg := range generatorPackages {
+		if err := sh.RunWith(flagEnv(), goexe, "generate", path.Join(packageName, pkg)); err != nil {
+			return err
+		}
+	}
+
+	dir, _ := os.Getwd()
+	c := codegen.NewInspector(dir)
+
+	if err := page_generate.Generate(c); err != nil {
+		return err
+	}
+
+	goFmtPatterns := []string{
+		// TODO(bep) check: stat ./resources/page/*autogen*: no such file or directory
+		"./resources/page/page_marshaljson.autogen.go",
+		"./resources/page/page_wrappers.autogen.go",
+	}
+
+	for _, pattern := range goFmtPatterns {
+		if err := sh.Run("gofmt", "-w", filepath.FromSlash(pattern)); err != nil {
+			return err
+		}
+	}
+
+	return nil
 }
 
 // Build hugo without git info