refactor(helpers): modernize reactive helpers and add tests

src/shared/lib/helpers/createVirtualizer/createVirtualizer.svelte.ts

@@ -291,7 +291,7 @@ export function createVirtualizer<T>(
                 },
             };
         } else {
-            containerHeight = node.offsetHeight;
+            containerHeight = node.clientHeight;
 
             const handleScroll = () => {
                 scrollOffset = node.scrollTop;

src/shared/lib/helpers/createVirtualizer/createVirtualizer.test.ts

@@ -56,6 +56,11 @@ function createMockContainer(height = 500, scrollTop = 0): any {
         configurable: true,
         writable: true,
     });
+    Object.defineProperty(container, 'clientHeight', {
+        value: height,
+        configurable: true,
+        writable: true,
+    });
     Object.defineProperty(container, 'scrollTop', {
         value: scrollTop,
         writable: true,