Refactor

[anilanar]

Refactors the original impl to do the following changes:

1. Converts polyfill to ponyfill. No more global side effects at module load. Exports enablePonyfill function.
2. enablePonyfill returns a cleanup function that undoes ponyfilling (WIP).
3. Converts to typescript. Improves some checks that is used for converting EventTarget -> Node -> Element -> HTMLElement using instance checks.
4. Improves scoping so that it works in iframes. Infers window/document from the passed scope instead of using the global window/document, which simply doesn't work for iframes.
5. Removes test infra (perhaps temporarily) which was source of many audit errors.
6. Released as @userlike/focus-visible.

[buscape]

lgtm in general, the only improvement we can think about is just using either focus-visible class or the data-focus-visible-added data attribute instead of both of them.

[anilanar]

@buscape I don't want to change what the original library does because chakra-ui relies on them and may rely on them in a different way in future.