Implement focusin/focusout events.

[sarenji]

What:

Adds the focusin and focusout events. Fixes #179.

Sorry this PR took so long, I needed some personal time and wanted to make sure I was informed. The focusin/focusout events are described in two documents:

• The official UIEvents documentation. Note: focusin/focusout bubble, unlike focus/blur, and fire before the target element receives focus.
• MDN's focusin/focusout pages. Notably, Firefox did not support the focusin/focusout events until 2017. But now every major browser supports the events.

Why:

I added document.addEventListener('focusin', handler); to some of my React components. In dom-testing-library, we can only trigger this handler by dropping down to the low-level fireEvent(element, event) API. For consistency, it'd be nice to provide a focusin/focusout convenience API like the other events.

How:

1. Changed tests to account for focusin/focusout.
2. Added the two events to events.js, and making sure they bubble.

Checklist:

• Documentation (N/A: The docs just link to the events.js file.)
• Tests
• Ready to be merged
• Added myself to contributors table

This is part of a bigger story around lack of focusin/focusout implementation: jsdom does not fire focusin/focusout events when focusing or blurring. And React does not expose focusin/focusout either.

Great library, BTW! Let me know if there's anything else I need to do. Thank you!

[kentcdodds]

Perfect! Thanks!

[kentcdodds]

🎉 This PR is included in version 3.16.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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