tpl: Allow the partial template func to return any type

This commit adds support for return values in partials.

This means that you can now do this and similar:

    {{ $v := add . 42 }}
    {{ return $v }}

Partials without a `return` statement will be rendered as before.

This works for both `partial` and `partialCached`.

Fixes #5783

tpl/template_info.go

@@ -22,10 +22,17 @@ type Info struct {
 	// Set for shortcode templates with any {{ .Inner }}
 	IsInner bool
 
+	// Set for partials with a return statement.
+	HasReturn bool
+
 	// Config extracted from template.
 	Config Config
 }
 
+func (info Info) IsZero() bool {
+	return info.Config.Version == 0
+}
+
 type Config struct {
 	Version int
 }