[event] add touchEvt support to localPnt, find owner svg. fixes #15

[hshoff]

Fixes: #15

Adds single arity support to localPoint(event) which will find the ownerSVG and loop for the owner on nested SVG. This use case was found by @conglei when his chart has nested svg icons that had event handlers. Thanks to @conglei for pointing this out.

You can still pass in a reference to a specific svg if you want your local point relative to a specific svg element by using dual arity localPoint(svgNode, event) or localPoint(event.target.ownerSVGElement, event).

Lastly, this adds touch event support to localPoint so you don't have to use touchPoint and localPoint. touchPoint will still exist to prevent breaking changes for folks. I also added touchPoint to the exports on index so you can now do import { touchPoint } from '@vx/event' if you want to.

Updated the /areas demo to support touch events and mouse events with the new single arity usage.

future will probably need a localTouchPoints that returns an array of local event.changeTouches points since localPoint returns the the first event.eventTouches and ignores the rest.