tpl: Rework to handle both text and HTML templates

Before this commit, Hugo used `html/template` for all Go templates.

While this is a fine choice for HTML and maybe also RSS feeds, it is painful for plain text formats such as CSV, JSON etc.

This commit fixes that by using the `IsPlainText` attribute on the output format to decide what to use.

A couple of notes:

* The above requires a nonambiguous template name to type mapping. I.e. `/layouts/_default/list.json` will only work if there is only one JSON output format, `/layouts/_default/list.mytype.json` will always work.
* Ambiguous types will fall back to HTML.
* Partials inherits the text vs HTML identificator of the container template. This also means that plain text templates can only include plain text partials.
* Shortcode templates are, by definition, currently HTML templates only.

Fixes #3221

deps/deps.go

@@ -20,7 +20,7 @@ type Deps struct {
 	Log *jww.Notepad `json:"-"`
 
 	// The templates to use.
-	Tmpl tpl.Template `json:"-"`
+	Tmpl tpl.TemplateHandler `json:"-"`
 
 	// The file systems to use.
 	Fs *hugofs.Fs `json:"-"`
@@ -40,7 +40,7 @@ type Deps struct {
 	Language *helpers.Language
 
 	templateProvider ResourceProvider
-	WithTemplate     func(templ tpl.Template) error `json:"-"`
+	WithTemplate     func(templ tpl.TemplateHandler) error `json:"-"`
 
 	translationProvider ResourceProvider
 }
@@ -158,7 +158,7 @@ type DepsCfg struct {
 
 	// Template handling.
 	TemplateProvider ResourceProvider
-	WithTemplate     func(templ tpl.Template) error
+	WithTemplate     func(templ tpl.TemplateHandler) error
 
 	// i18n handling.
 	TranslationProvider ResourceProvider