[Combobox unique value]: New value dev implementation

[thyhmdo]

Ref link for implementation #13903

Acceptance Criteria

Allow users to type in a new value in the ComboBox and accept it as a new value

• Add a new prop to enable the behavior
• Typing and hitting enter should no longer clear the input as described in the spec: [Combobox unique value]: Design spec pass 2 (dev and a11y) #13903 (comment)
• Evaluate if we need to modify the selectedItem state, but users could probably get the value without this even using target.value in the onChange callback if we have a ref on the input

[tay1orjones]

For this it seems the primary piece is to update the component to not clear the input after hitting enter if there is not a matched element in the listbox.

There was a question of if we needed to wait on this until we refactored ComboBox off of <Downshift> to instead use useComboBox #14684, but after some discussion we think the refactor shouldn't be necessary for this.