Refactor and reduce code to allow for a smaller bundle size

Supporting added ResizeObserver features and improving the library, has increased the bundle size. As the specification is now at a stable state, it could be a good time to refactor and streamline the code.

**Size:** 2.8KB, minified and gzipped.
**Goal:** <=2.5KB, minified and gzipped.

https://bundlephobia.com/result?p=@juggle/resize-observer
