"Add a new default color" button on the chart settings form is a silent no-op (wrong #array_slicing_args offset)

Problem/Motivation

Clicking "Add a new default color" on a chart's Display settings does nothing — the AJAX request completes, the form rebuilds, but no new color row is added. The "Remove" button next to existing rows works fine, only "Add" is broken.

Steps to reproduce

1. Create a new simplytest.me project for this project (Charts).
2. Enable a chart library submodule (e.g. "Chart.js Charts") and set it as the default library at /admin/config/content/charts.
3. In the same form's Display settings, find the default colors section.
4. Click "Add a new default color".
5. Observe: no new row appears.

Root cause

In src/Element/BaseSettings.php::processConfigForm(), the "remove" and "add" buttons both derive $element_parents from #array_parents via #array_slicing_args. They sit at different nesting depths — "remove" is 4 segments deep (display/colors/$index/remove), "add" is 5 segments deep (display/colors/_add_new/add_item/submit) — but the "add" button reuses the "remove" button's length => -4 instead of -5. This makes addDefaultColorElementItemSubmit() write the new _new index to the wrong element-state branch, one defaultColorElementItemsAjax() never reads back.

Proposed resolution

Change '#array_slicing_args' => ['offset' => 0, 'length' => -4] to -5 for the "add" button. Patch attached, tested against 5.2.2.