Add requestIdleCallback feature (#2274)

Co-authored-by: Daniel D. Beck <[email protected]>

Author: autonome

features/draft/spec/requestidlecallback.yml

@@ -0,0 +1,10 @@
+name: requestIdleCallback()
+description: "The `requestIdleCallback()` API queues a function that runs in idle browser time, either at the end of a frame or when the user is inactive. Also known as the background tasks API."
+spec: https://w3c.github.io/requestidlecallback/
+caniuse: requestidlecallback
+compat_features:
+  - api.IdleDeadline
+  - api.IdleDeadline.didTimeout
+  - api.IdleDeadline.timeRemaining
+  - api.Window.cancelIdleCallback
+  - api.Window.requestIdleCallback