fix: Add a hedge in case we ever use Jest with Storybook.

See storybookjs/storybook#18399

We don't need this at the moment, but I discovered this while looking
into another Storybook issue, and it seems worth adding now in case we
use Jest in the future, as it is not well documented. (Also, it sounds
like Storybook projects created with recent versions will
automatically add this file.)

.storybook/preview-head.html

@@ -0,0 +1,5 @@
+<script>
+    // https://github.com/storybookjs/storybook/issues/18399
+    // Fix: add `jest` to global
+    window.global = window;
+</script>