[compiler:playground] JS tab is expanded by default

When using the playground you typically want to see what it outputs, so
let's make the JS tab expanded by default.

ghstack-source-id: 721bc4c381
Pull Request resolved: https://github.com/facebook/react/pull/29203

compiler/apps/playground/__tests__/e2e/page.spec.ts

@@ -28,7 +28,6 @@ test("editor should compile successfully", async ({ page }) => {
   });
 
   // User input from hash compiles
-  await page.getByRole("button", { name: /JS/ }).click();
   await page.screenshot({
     fullPage: true,
     path: "test-results/01-show-js-before.png",

compiler/apps/playground/components/Editor/Output.tsx

@@ -177,7 +177,7 @@ function getSourceMapUrl(code: string, map: string): string | null {
 }
 
 function Output({ store, compilerOutput }: Props) {
-  const [tabsOpen, setTabsOpen] = useState<Set<string>>(() => new Set());
+  const [tabsOpen, setTabsOpen] = useState<Set<string>>(() => new Set(['JS']));
   const [tabs, setTabs] = useState<Map<string, React.ReactNode>>(
     () => new Map(),
   );