problems(js): add “2621. Sleep”

Signed-off-by: Matej Focko <me@mfocko.xyz>

problems/sleep.js

@@ -0,0 +1,13 @@
+/**
+ * @param {number} millis
+ */
+async function sleep(millis) {
+    return new Promise((resolve, reject) => {
+        setTimeout(() => resolve('Slept enough!'), millis);
+    });
+}
+
+/**
+ * let t = Date.now()
+ * sleep(100).then(() => console.log(Date.now() - t)) // 100
+ */