fix(combobox): render ellipsis when label is overflowing

[janhassel]

Closes #11934

Changelog

New

• Added text-overflow: ellipsis to listbox label

Testing / Reviewing

• In the combobox story, select the first option ("An example option..."). Should truncate with ellipsis when the combobox is closed.
• Note that when the combobox is opened, the label is a text input andshould therefore not truncate with ellipsis as the user can move the input caret.

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