add `virtual` to `ValFunction.BindAndCopy`

[symful]

It will allow the usage to override the use of bind function to allow the implementation of forever bind (not allowing any new binding to the function for the sake of not losing locals context on whatever uses developers want).

class ValBFunction(Function function, ValMap outerVars) : ValFunction(function, outerVars) {
    public override ValFunction BindAndCopy(ValMap contextVariables) {
        return this;
    }
}