extensions: improve action items bar accessibility

Created by: tjkandala

Part of #36751 (closed). Closes #36850 (closed), #36852 (closed), #36854 (closed), and #36855 (closed).

• Use aria-disabled instead of disabled for action items so they can retain focus when loading and so screen readers will read their labels.
• Use <div> instead of lists for single children.
• Add keyboard shortcut to enter action items bar to aria-label for toggle.

Test plan

• Existing automated tests pass
• Navigate to action items bar toggle with keyboard w/ screen reader on
  • 
  • Ensure that screen reader announces keyboard shortcut to enter panel
  • Press down/up arrow keys until you reach a disabled action item
    • Ensure that screen reader announces label.
  • Press down/up arrow keys until you reach an enabled action item with an asynchronous action (e.g. toggle git blame)
    • Press enter to run action
    • Ensure that focus stays on the action during and after execution

App preview:

• Web
• Storybook

Check out the client app preview documentation to learn more.