Explicitly allow a base importer without a base URL

[nex3]

See sass/dart-sass#2208. This already implicitly throws an error per spec due to trying to do WHATWG URL resolution with a null base, but the spec is not explicit about this and in practice Dart Sass doesn't throw an error.

• Update specification (Clarify that base importer resolution does not require a URL #3832)
• Tests (Add a test for an importer without a base URL sass-spec#1972)
• Delete StringOptionsWithoutImporter and StringOptionsWithImporter.

[nex3]

Per sass/dart-sass#2208 (comment), we've decided not to deprecate this after all. Instead, I'll update the specification to explicitly allow it.