[v1] `loadOnce()` as the single directive about assets

[promatik]

This looks promising!

I think we should not rely on dots or slashes, because a path may be just @loadOnce('main.js'), we should rely on the name ending with .js or .css, and expect users to not use ids that look like a file name.

Maybe we could also check if file exists, but it's more "expensive" probably and in a situation where the file mistakenly doesn't exist it would be, wrongly, expect an ending block – so it's not a good idea.