Feature request: support for handlebars templates

Handlebars templates are currently not supported.

There are two ways to add comments to a template. The first is to use the template language's own comments, which for Handlebars look like:

```
{{!--
Copyright 2025 My Name
--}}
```

The second is to write them in the template's target language, ensuring they survive the template process. For example, in a file `index.html.hbs`, it would use HTML comments. I'd suggest defaulting to Handlebars comments, with a command-line flag to enable target language comments.

For an MVP, supporting Handlebars comments only is enough.