d: fix interface syntax error

Fix syntax error regarding interface inheritance of the Lexer. It
appeared when the `%code lexer` option was used.

Reported by ledaniel2.
<https://github.com/akimd/bison/issues/84>

* data/skeletons/lalr1.d: Fix syntax.
* tests/d.at: Test it.

data/skeletons/lalr1.d

@@ -269,9 +269,10 @@ b4_user_union_members
     }
   }]])[
 
-]b4_lexer_if([[  private class YYLexer implements Lexer {
+]b4_lexer_if([[private class YYLexer: Lexer
+{
 ]b4_percent_code_get([[lexer]])[
-  }
+}
 ]])[
   /** The object doing lexical analysis for us.  */
   private Lexer yylexer;