Cross-link to new imports spec (#4111)

Author: mit-mit

working/0649 - Import shorthand/proposal.md

@@ -1,3 +1,5 @@
-This document [has moved].
+This 'Import Shorthand Syntax' document [has moved] to the archive.
+It has has been succeeded by the [Unquoted imports] document.
 
 [has moved]: https://github.com/dart-lang/language/blob/main/archive/0649%20-%20Import%20shorthand/proposal.md
+[Unquoted imports]: https://github.com/dart-lang/language/blob/main/accepted/future-releases/unquoted-imports/feature-specification.md