dropdown performance

[s-cork]

Describe the bug
try creating a dropdown with 1000+ items

• currently it takes a long time to create the items (slow initial loading)
• when showing the items, it takes a while show the items (should feel instant)

Possible solution

• use virtualization
  • only render what the user can see
  • don't instantiate the anvil components until we need them

Additional Context

• anvil-extras added virtualization support, we can probably use a similar implementation here
  https://github.com/anvilistas/anvil-extras/blob/main/client_code/virtualize.py