Add support for safe navigation operator (elvis operator)

[navrocky]

I'll try to explain my thoughts with the help of examples.

The variable var will be null if there is no obj key:

## set var = obj?

The variable var will be null if there is no obj key or obj exists but no obj.key exists:

## set var = obj?.key?

The variable var will be null if there is no obj key or throws if there is no obj.key exists:

## set var = obj?.key