new(xychart): make DataProvider optional

[williaster]

🚀 Enhancements

This PR adds a usability improvement to @visx/xychart, making it optional for package consumers to render DataProvider which is arguably an internal implementation detail (note that it is already optional for consumers to render TooltipProvider and EventEmitterProvider).

Users can still render DataProvider up the parent chain for advanced use cases like linked charts, where it's desirable to share a single DataContext.

// before (still valid)
() => (
  <DataProvider xScale={..} yScale={..} theme={..}>
    <XYChart width={..} height={..}>
      {...}
    </XYChart>
  </DataProvider>
)

// after
() => (
  <XYChart 
    xScale={..} 
    yScale={..} 
    theme={..}
    width={..} 
    height={..}
  >
    {...}
  </XYChart>
)

Testing

• new jest tests
• verified that /xychart demo is still functional

[williaster]

💬 only DataProvider was removed, everything else in this file is a de-indentation

[williaster]

💬 this throws only because nimbus transforms console.warns to throw

[coveralls]

Pull Request Test Coverage Report for Build 344801060

• 6 of 7 (85.71%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.09%) to 60.182%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
packages/visx-xychart/src/components/XYChart.tsx	6	7	85.71%

Totals
Change from base Build 344463151:	0.09%
Covered Lines:	1554
Relevant Lines:	2451

---

💛 - Coveralls