d: add yyerrok

In D's case, yyerrok() is a private method of the Parser class.
It can be called directly as `yyerrok()` from the grammar rules section.

* data/skeletons/lalr1.d: Add yyerrok().
* examples/d/calc/calc.y, examples/d/simple/calc.y: Demonstrate yyerrok().
* tests/calc.at: Update D tests to use yyerrok().

examples/d/calc/calc.y

@@ -53,7 +53,7 @@ input:
 line:
   EOL
 | exp EOL           { writeln ($exp); }
-| error EOL
+| error EOL         { yyerrok(); }
 ;
 
 exp:

examples/d/simple/calc.y

@@ -51,7 +51,7 @@ input:
 line:
   EOL
 | exp EOL           { writeln ($exp); }
-| error EOL
+| error EOL         { yyerrok(); }
 ;
 
 exp: