[Combobox] Append custom values to top of Combobox dropdown, not bottom

[macandcheese]

Check existing issues

• [x] I have checked for existing issues to avoid duplicates

Description

Currently, when allow-custom-values is true and a user adds a custom value, it is appended to the bottom of the Combobox dropdown.

This can be confusing, as when presented alongside groups, it's not clear if the added value is part of the last group due to indentation:

Acceptance Criteria

The appended items are added to the top

Relevant Info

There are probably additional desirable configurations - sometimes an app may want to add a created item to a specific group, or allow a user to remove an item they've created.

cc @ashetland @mitc7862

Which Component

Combobox

Example Use Case

No response

Priority impact

p4 - not time sensitive

Calcite package

• [X] @esri/calcite-components
• [ ] @esri/calcite-components-react

Esri team

Calcite (design)

[github-actions[bot]]

cc @geospatialem, @brittneytewks