Remove `!` operator for consistency

One must use `not`

src/language/PEGGrammar.hpp

@@ -128,7 +128,7 @@ struct unary_minusminus : TAO_PEGTL_STRING("--") {};
 struct unary_plus :  one< '+' > {};
 struct unary_minus : one< '-' > {};
 
-struct unary_not : sor< one< '!'> , not_kw > {};
+struct unary_not : not_kw {};
 
 struct unary_operator : seq< sor< unary_plusplus, unary_minusminus, unary_plus, unary_minus, unary_not>, ignored > {};