feat: add Avatar support for ComboBoxItem and PickerItem (#8931)
* feat: add Avatar support for ComboBoxItem and PickerItem
For the new `avatar` style macro, I copied a subset of ComboBox's `image` style macro.
Avatar controls size via its `size` prop, so I didn't include that in the style macro.
* feat: avatar support for Picker button, render for AvatarContextValue
* Add center baseline to Avatar by default
* Simplify s2-docs for ComboBox and Picker with Avatars
* Add defaultValue and placeholder to new ComboBox/Picker docs
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Co-authored-by: Dylan Kario <kario@adobe.com>
Co-authored-by: Daniel Lu <dl1644@gmail.com>
Co-authored-by: Devon Govett <devongovett@gmail.com>
Co-authored-by: Yihui Liao <44729383+yihuiliao@users.noreply.github.com>