use `html/template` to parse template files

Why not use the [`html/template`](https://pkg.go.dev/html/template) package to parse template files? `text/template` is not safe for HTML, it can cause HTML injection.

https://github.com/yaitoo/xun/blob/260cebf5077de7b4d9bfb32dd4cb1672a2c59368/html_template.go#L7