Change window to globalThis in Virtualizer.ts for SSR

.changeset/rare-actors-grab.md

@@ -0,0 +1,5 @@
+---
+'@lit-labs/virtualizer': major
+---
+
+Change window to globalThis in Virtualizer.ts to support SSR scenarios.

packages/labs/virtualizer/src/Virtualizer.ts

@@ -34,7 +34,8 @@ import {ScrollerController} from './ScrollerController.js';
 // polyfill in the package; this bit of module state facilitates
 // a simple mechanism (see ./polyfillLoaders/ResizeObserver.js.)
 // for loading the polyfill.
-let _ResizeObserver: typeof ResizeObserver | undefined = window?.ResizeObserver;
+let _ResizeObserver: typeof ResizeObserver | undefined =
+  globalThis?.ResizeObserver;
 
 /**
  * Call this function to provide a `ResizeObserver` polyfill for Virtualizer to use.