JSONata leaks internal references in expression when using lambdas

I am creating this after corresponding with @andrew-coleman over email as it has some security implications.

Currently, one can use lambdas as a way to grab references to JSONata internals. For example:

```
$leak_env_refs := function() {1};
$leak_env_refs.environment.bind('whatever_you_want', false);
```

This seems unintentional and can potentially be used maliciously.