feat: support optional array, string access and deep nil coalescing

[ckganesan]

This PR is raised to add support for optional string and array access. #540 #547 #573

[antonmedv]

This is not exactly correct behavior I'm looking for.

In case array?.[7] operator ?. checks what array itself defined and not nil.

So the code

array?.[7]

will panic in there is no 7 element index.

What we can do is to alter [7] behavior with operator ?. after it.

array[7]?.field

In this case we can alter OpFetch and do not panic if 7 index does not exist.

This mean we, unfortunately, does not have a syntax to alter [7] behavior if it is the last postfix op.

But we can alter the whole "chain" in presence of ?? operator!

array[7].field ?? 'default'