ebay-chips-combobox: add custom text as an item to listbox

[ArtBlue]

Feature Request

During the chips-combobox Applause a11y testing, we tested a feature we thought would improve the UX - adding the typed entry as the first item with a helper text of "(add new)." The testing confirmed it made a notable difference.

Description

1. Add the custom typed text as the first item in the listbox.
2. Add a helper text of "(add custom)" as was indicated by the conclusion of the Applause testing.

Context

This helps both visual and screen reader users to get a better context around the specific interaction of adding a new item.

Screenshots

Now

Desired Pattern

Optimal Pattern

Since listbox already has a pattern for a secondary text, this would be the ideal use with no additional structural adjustments: