tpl/transform: Make Plainify and ToMath return template.HTML

None of these are useful as plain strings in the templates, which forces the users to do `transform.Plainify "foo" | safeHTML`.

If people have trust issues with the output of these functions, they need to just stop using them.

Closes gohugoio#8732

docs/content/en/functions/transform/Plainify.md

@@ -6,7 +6,7 @@ keywords: []
 action:
   aliases: [plainify]
   related: []
-  returnType: string
+  returnType: template.HTML
   signatures: [transform.Plainify INPUT]
 aliases: [/functions/plainify]
 ---

tpl/transform/transform.go

@@ -188,18 +188,18 @@ func (ns *Namespace) Markdownify(ctx context.Context, s any) (template.HTML, err
 }
 
 // Plainify returns a copy of s with all HTML tags removed.
-func (ns *Namespace) Plainify(s any) (string, error) {
+func (ns *Namespace) Plainify(s any) (template.HTML, error) {
 	ss, err := cast.ToStringE(s)
 	if err != nil {
 		return "", err
 	}
 
-	return tpl.StripHTML(ss), nil
+	return template.HTML(tpl.StripHTML(ss)), nil
 }
 
 // ToMath converts a LaTeX string to math in the given format, default MathML.
 // This uses KaTeX to render the math, see https://katex.org/.
-func (ns *Namespace) ToMath(ctx context.Context, args ...any) (string, error) {
+func (ns *Namespace) ToMath(ctx context.Context, args ...any) (template.HTML, error) {
 	if len(args) < 1 {
 		return "", errors.New("must provide at least one argument")
 	}