[Enhancement] Fix the create button at the bottom of the dropdown list

[jarekmorawski]

Related to #35851 -- some work on #35851 deals with the AC in this issue

We noticed in user testing that some people couldn't figure out how to add a new item in the dropdown component. In addition to the existing logic that allows users to type the name and select an option from the list to create a new item, we want to add a fixed button that will be visible at all times.

fixedcreateitem.mov

Acceptance criteria

• There's a new item fixed at the bottom of the list. It is visible to users as they scroll up and down the list.
• Users can quickly navigate to it using the Tab key.
• The button is Gutenberg-Blue (it's now Gutenberg-900) and says + Create new.
• Clicking the button or pressing Enter opens the item creation modal.
• When users start typing, the button's label changes to + Create "[phrase]" just like it does now.

[mdperez86]

Newly created item is placed at the bottom of the list.

@jarekmorawski this is not possible if we apply some sorting criteria to the list like: by name, or anything different to the id

[octaedro]

Hey @jarekmorawski,
Considering this comment is it ok if we remove that AC (Newly created item is placed at the bottom of the list)?

[jarekmorawski]

Removed. ✔️

[nathanss]

I created the issue #37043, I think we should migrate to the new TreeControl component before implementing this to avoid rework.

[nathanss]

Closed by #37479