RichText: List: Fix outdent with children

[ellatrix]

Description

After #12667, outdenting list items with children became broken. The children should also be indented.

How has this been tested?

Start with the following content:

<!-- wp:list -->
<ul><li>A<ul><li>B<ul><li>C</li><li>D</li></ul></li></ul></li></ul>
<!-- /wp:list -->

Set the caret at "B", then press the outdent button. "C" and "D" should also be one level outdented.

Screenshots

Types of changes

Bug fix.

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.

[youknowriad]

Works well in my testing.

[ellatrix]

Thanks!

[mcsf]

Starting with the following list:

<!-- wp:list -->
<ul><li>A<ul><li>B<ul><li>C</li></ul></li><li>D</li></ul></li></ul>
<!-- /wp:list -->

Place the caret on D, then decrease indentation.
Observed: nothing changes.
Expected: for D to return to level 0, like so:

<!-- wp:list -->
<ul><li>A<ul><li>B<ul><li>C</li></ul></li></ul></li><li>D</li></ul>
<!-- /wp:list -->

[ellatrix]

Thanks @mcsf! I'll follow up with a PR.

[mcsf]

Conversely, starting at:

<!-- wp:list -->
<ul><li>A<ul><li>B<ul><li>C</li></ul></li></ul></li><li>D</li></ul>
<!-- /wp:list -->

Place the caret at D, then increase indentation.
Observed: D jumps from level 0 to 2.
Expected: D jumps to level 1.

[ellatrix]

The first one is fixed by #13562. Looking into the other one now.