Rework nested dropdown tree

[qroll]

Changes

• [delete] branch

Reworks the nested dropdown for floating-ui (related to #443). A temporary story is included to test the new component

Separate PRs will be created for migrating the NestedSelect and NestedMultiSelect parents

• render the tree as a list
  • the current dropdown uses the map data structure and recursion to process/render the dropdown list items
  • the proposed approach converts the tree structure into a flattened list
  • this is done in implementations like Fluent UI and react-virtualized-tree
  • pros/cons:
  • ➕ potential for virtualisation
  • ➕ refs are much easier to manage as they just need to be pushed into an array
  • ➕/➖ some operations are easier, some harder
  • ➖ might be less intuitive dealing with trees without recursion
  • ➖ have to manage visibility of subitems yourself
  • I think it's still overall easier to manage a flattened list (rendering list items is easier via iteration vs recursion)
• mirror the focus and keyboard behaviour in the non-nested dropdown list
• some accessibility rework
  • apply the tree and treeitem roles
  • replace buttons, in line with recommendations in the APG to keep a single tab stop (and also to avoid spamming the Form Controls menu in Voiceover)
  • use left/right arrow keys to collapse/expand subtrees