When we try to increase / decrease an indentation level of a selection, just change
indentation level of each block in the selection, and then make sure all styles
are consistent via reformatList.
This actually fixes indentation of selection: previously all blocks had the same indentation after this action.
Add a unit test that covers as much weird selections & indentation cases as I could imagine.
Slightly overlaps with D29208: [NestedListHelper] Improve indentation code, so this one should be applied after that.