fix(ui5-li): implement edit mode for keyboard navigation in list items

Implements an edit mode for list items following the SAP Fiori
Keyboard Interaction spec (Intentional Edit Pattern, p86-87):

- **Navigation mode** (default): Tab/Shift+Tab forwards out of the
  list. Arrow keys navigate between items.
- **Edit mode** (toggled by F2 or F7): Tab flows through inner
  focusable elements within an item, then continues to the next
  item's elements, and exits the list after the last item.
  Shift+Tab flows backward symmetrically. Arrow Up/Down transfers
  focus to the same-index element in adjacent items.

The delete button in Delete selection mode is now keyboard
accessible — reachable via edit mode Tab flow without requiring
the Delete key shortcut.

Fixes #13220

Author: kgogov