fix(v2/Dropdown): allow controlled selectedItems, render icon in MultiSelectItem

[karrui]

Problem

This PR cherry picks a bunch of SingleSelect/MultiSelect fixes that were encountered whilst working on #3556.

Solution

Features:

• feat(Select): allow setting of empty placeholder
• feat(MultiSelectItem): show icon in tag if it exists

Bug Fixes:

• fix(ComboboxClearButton): show active state if item has been selected
  • Previously, the clear button will remain in inactive styling even if item has been selected
• fix(Single/MultiSelectProvider)!: allow control of selectedItem, refilter items whenever items prop changes
  • Instead of using initialSelectedItem prop, allow users of the component to pass in current value for selection.
• fix(SingleSelectProvider): use label string as aria-live announcement
  • the value was used previously, which may cause screenreaders to read something different that what is being rendered.

Before & After Screenshots

Most stories should not have a change. The only changes should be the ones for rendering an icon in the selected items in MultiSelect.

[tshuli]

minor nits but lgtm otherwise!