The canonical URL of the file that contains the load, if that information is available.
For an Importer, this is only passed when the url parameter is a
relative URL or when its URL scheme is included in Importer.nonCanonicalScheme. This ensures that canonical URLs are always
resolved the same way regardless of context.
For a FileImporter, this is always available as long as Sass knows the canonical URL of the containing file.
Whether this is being invoked because of a Sass
@import rule, as opposed to a @use or @forward rule.
This should only be used for determining whether or not to load import-only files.
Contextual information passed to Importer.canonicalize and FileImporter.findFileUrl. Not all importers will need this information to resolve loads, but some may find it useful.