Refactor Schedule, remove React-isms

Once the API stabilizes, we will move Schedule this into a separate
repo. To promote adoption, especially by projects outside the React
ecosystem, we'll remove all React-isms from the source and keep it as
simple as possible:

- No build step.
- No static types.
- Everything is in a single file.

If we end up needing to support multiple targets, like CommonJS and ESM,
we can still avoid a build step by maintaining two copies of the same
file, but with different exports.

This commit also refactors the implementation to split out the DOM-
specific parts (essentially a requestIdleCallback polyfill). Aside from
the architectural benefits, this also makes it possible to write host-
agnostic tests. If/when we publish a version of Schedule that targets
other environments, like React Native, we can run these same tests
across all implementations.

packages/react-reconciler/src/ReactFiberScheduler.js

@@ -153,7 +153,7 @@ import {
 import {Dispatcher} from './ReactFiberDispatcher';
 
 export type Deadline = {
-  timeRemaining: () => number,
+  timeRemaining(): number,
   didTimeout: boolean,
 };
 

packages/schedule/index.js

@@ -3,8 +3,6 @@
  *
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
- *
- * @flow
  */
 
 'use strict';