This allows @import statements to be resolved in a set of user provided resources (e.g. from resources.FromString or css.ChromaStyles) before the assets filesystem.
The option also applies to css.PostCSS via the shared import inlining, and css.Sass requires the dartsass transpiler. The import context is part of the transformation cache key.
Fixes#15103
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>