null Safe select operator, aka. Optional_chaining

[He-Pin]

refs: google/jsonnet#930
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Optional_chaining#optional_chaining_with_expressions

In JSONPath, we can select deeply, even it may be null.

  maybeNull: someObject?.innerObject?.whatIWant

I think we can support this with a ?. instead of ..

I have implemented it with :

      eval(
        """
          |local obj = {a: 1};
          |local obj2 = null;
          |obj2?.a
          |
          |""".stripMargin) ==> ujson.Null

or maybe just use std.get

This project is using this syntax https://github.com/jam01/xtrasonnet

[jam01]

#124 for reference