Appending to a list should maintain list

[mvorisek]

Description

repro: https://3v4l.org/fV80Z

I know historically there is internal "next free index variable", but array with 0...n-1 keys should stay list even when some key/value is added. using [] operator.

Related with phpstan/phpstan#11628 as array_is_list($v) should imply list and such list must maintain list behaviour when value is appended/unset to/from the end.

[nielsdos]

This is a behaviour change with possible BC concerns. Therefore this needs an RFC.