Add support for customizing the [importmap](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/script/type/importmap). Currently, it's hardcoded[^1] to only import the latest version of Hotwired Stimulus. [^1]: https://github.com/levkk/rum/blob/9575f23d289d74f978937bf3805818230f552186/rwf/src/view/template/head.html#L4-L6
