Add `??` with nullsafe or default value.

[He-Pin]

Motivation:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Nullish_coalescing

We need to something like maybeNullObj?['someField'] ?? 'defaultValue'

refs: #327

or we can just implement mvel:

person?.address?.city?: "Unknown City"

[CertainLach]

Note that in JS (and in jrsonnet) it is not maybeNullObj?['someField'], but maybeNullObj?.['someField'] for bracket index expression

[He-Pin]

as this can be implemented in future version, we plan to just using functions for now.

[He-Pin]

I found this https://github.com/jam01/xtrasonnet project is using this synatax.