initial support for is null and is not null

will nedd some work, but just for now

parser.cpp

@@ -472,6 +472,12 @@ namespace usql {
 			    return RelationalOperatorType::lesser;
 		    case TokenType::lesser_equal:
 			    return RelationalOperatorType::lesser_equal;
+		    case TokenType::is:
+		    	    if (m_lexer.tokenType() == TokenType::keyword_not) {
+					m_lexer.skipToken(TokenType::keyword_not);
+			    		return RelationalOperatorType::is_not;
+			    }
+			    return RelationalOperatorType::is;
 		    default:
 			    throw Exception("Unknown relational operator " + op.token_string);
 	    }