tpl: Add templates.Current

This commit also

* Unexport all internal state in TemplateInfo.
* Make the dispatcher keys used for passing context.Context into uint8 from string to save memory allocations.

Co-authored-by: Joe Mooring <joe@mooring.com>

Updates #13571

common/hugo/hugo.go

@@ -139,9 +139,13 @@ func (i HugoInfo) IsMultilingual() bool {
 	return i.conf.IsMultilingual()
 }
 
-type contextKey string
+type contextKey uint8
 
-var markupScope = hcontext.NewContextDispatcher[string](contextKey("markupScope"))
+const (
+	contextKeyMarkupScope contextKey = iota
+)
+
+var markupScope = hcontext.NewContextDispatcher[string](contextKeyMarkupScope)
 
 type Context struct{}