script compare with negative number exception

[jiezenghuang]

version: 4.6.1

example as below：

static const char* xml_text = R"(
 <root BTCPP_format="4">
     <BehaviorTree>
        <Sequence>
            <Script code=" A:=0 " />
            <AlwaysSuccess _failureIf="A!=-1"/>
        </Sequence>
     </BehaviorTree>
 </root>
 )";

int main(int argc, char** argv)
{
    BT::BehaviorTreeFactory factory;

    try
    {
        auto tree = factory.createTreeFromText(xml_text);
        tree.tickWhileRunning();
    }
    catch(const std::exception& e)
    {
        std::cerr << e.what() << '\n';
    }  

    return 0;    
}

output：
Error in the script "A!=-1"
error: while parsing Grammar::Expression
|
1 | A!=-1
| ^ operator cannot be mixed with previous operators

[Aglargil]

@facontidavide
#896
The content of this PR may resolve this issue.