Deprecate the use of fragments in Sass loads

[nex3]

Currently, Sass loads allow arbitrary URLs, including URL fragments. However, per spec fragments are not supposed to affect which resource is indicated by the URL, which means that importers in principle should not handle them differently. In addition, the algorithm for determining the default namespace ignores the fragment, meaning that if imports do treat fragments as part of the resource identifier it can lead to confusing behavior as described in sass/dart-sass#1635.

To avoid this confusion and semantic problem, I believe we should deprecate and eventually forbid URL fragments in Sass load strings.