Add hint when found cc language import in repl

Author: Linyxus

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -3706,7 +3706,11 @@ object Parsers {
           in.languageImportContext = in.languageImportContext.importContext(imp, NoSymbol)
           for case ImportSelector(id @ Ident(imported), EmptyTree, _) <- selectors do
             if Feature.handleGlobalLanguageImport(prefix, imported) && !outermost then
-              syntaxError(em"this language import is only allowed at the toplevel", id.span)
+              val hint =
+                if ctx.mode.is(Mode.Interactive) then
+                  f"\nTo use this language feature, include the flag `-language:$prefix.$imported` when starting the REPL"
+                else ""
+              syntaxError(em"this language import is only allowed at the toplevel$hint", id.span)
             if allSourceVersionNames.contains(imported) && prefix.isEmpty then
               if !outermost then
                 syntaxError(em"source version import is only allowed at the toplevel", id.span)