Replace some tools functions with prototype calls

[arthuro555]

I was sondering, wouldn't it be worth it to instead of making a lot of useless functions like

      export const getVariableNumber = function (
        variable: gdjs.Variable
      ): number {
        return variable.getAsNumber();
      };

couldn't we just call the method from the prototype (instead of using as function gdjs.evtTools.common.getVariableNumber, we could use gdjs.Variable.prototype.getAsNumber.call for the same result). This would remove unnecessary API surface that we need to maintain, and make the code size smaller.