Tree LOD with virtualization demo opens with error in CodeSandbox due to "@lit-labs/virtualizer" import

[ddaribo]

Steps:

1. Open the Load On Demand With Virtualization demo in CodeSandbox.
2. Oberve the error.
3. Delete line 8 (import "@lit-labs/virtualizer";)
4. Sample compiles.

The error states:
Failed to execute 'define' on 'CustomElementRegistry': the name "lit-virtualizer" has already been used with this registry.

However, when running the sample locally, this line is needed, so that the virtualization feature works.

The sample was introduced with this PR.