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.