feat: add option to set element class on hover

[rChaoz]

Important: I have two different changes in the same PR as I want to use the version generated by codesandbox, but when ready to mege this can be fixed.

What:

1. Add option to set a class on target element on hover. Useful for testing hover states either by manually adding .my-hover-class on top of :hover in CSS or with a plugin such as the one for PostCSS or Storybook.
2. Fix focus event not sent sometimes. startSelecting() is called before focusElement(). Setting a selection on an element sets that element as the document.activeElement, causing focusElement() not to do anything.

Why:

Allow testing of CSS hover states using userEvent.hover / userEvent.unhover or other commands that move the mouse.

How:

Add a new config option hoverClass (string) which defaults to null. When set (non-null), target.classList.add(hoverClass) is called whenever mouseenter is emitted and target.classList.remove(hoverClass) whenever mouseleave is emitted.

Checklist:

• [ ] Documentation
• [ ] Tests
• [x] Ready to be merged

[codesandbox-ci[bot]]

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