Assumes --base parameters or BASE_URL environment variables are always provided with trailing slash. This is reasonable, since base-path-based hosting will only work within actual sub-paths, which must end with a /. Else they would be resources or documents. It's their literal definition.
For items in content collections this generates absolute URLs for hrefs with using the url () supply function at time of loading.
Does not make assumptions about where in the path the root or the sitemap are found.
Adds the option to build URLs with index.html suffix.
Converges on the idea of being able to call the root slug / by its name, even though it is not part of slug-space.
