new(event): support FocusEvents in localPoint

[williaster]

🚀 Enhancements

This adds support for FocusEvents to @visx/event's localPoint and adds corresponding tests. This is needed to enable tab-able / focusable tooltips in XYChart (coming in another PR).

Implementation-wise, since FocusEvents do not have a clientX/Y like MouseEvents, I think it makes sense to have the location of the focus event be the center of the event target's bounding client rect but am open to other thoughts.

@kristw @hshoff

[coveralls]

Pull Request Test Coverage Report for Build 399280339

• 6 of 6 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.07%) to 61.446%

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Totals
Change from base Build 399258596:	0.07%
Covered Lines:	1718
Relevant Lines:	2627

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💛 - Coveralls