Implements various smaller combobox improvements

[haukesomm]

This PR implements various smaller improvements for the headless combobox. Check out the linked individual commits below to see the changes in smaller, more concise diffs:

• Adds a hook to control whether the combobox's dropdown opens lazily or eagerly
  • i.e.: When the user starts typing vs as soon as the user focuses the input
• Reduces flickering when re-opening a combobox
• Adds missing documentation regarding the combobox's selection mode