Add playground.

Author: trekhleb

src/playground/__test__/playground.test.js

@@ -0,0 +1,5 @@
+describe('playground', () => {
+  it('should perform playground tasks', () => {
+    // Place your playground tests here.
+  });
+});

src/playground/playground.js

@@ -0,0 +1 @@
+// Place your playground code here.