Add support for shortcut operators for lists in painless

[ctindel]

Following up on this bug report:

#21026

Just creating a feature request to allow shortcut operators for lists in painless.

[jdconrad]

Note as an example this would be like the following:

def x = new ArrayList(); def y = 1; x += y;

The list as a result would be [1].

[jdconrad]

We have decided to not change this. Closing.