Update JS evaluator to read back variables

Although scripts can update WF variables, we never write them back into WF variables (unless the user does so explicitly using the `setVariable` function.

We could implement this like this (pseudo code):

```csharp
// Before script execution, we set all WF variables (nothing to change here).
// After script execution, read back variables and update WF variables:

foreach(var variable in workflowVariables)
{
var updatedValue = engine.GetValue(variable.Name);
workflowContext.SetVariable(variable.Name, updatedValue);
}
```

_Originally posted by @sfmskywalker in https://github.com/elsa-workflows/elsa-core/discussions/2446#discussioncomment-1681417_