new(visx/visx): export @visx/xychart package

[williaster]

🚀 Enhancements

This exports @visx/xychart from the umbrella @visx/visx. Closes #974 , supersedes #984.

Note that this export is name-spaced under the XYChart.* object because of name conflicts with other packages. For 2.0.0 we could make this consistent across all packages (e.g., @visx/annotation exported under the Axis object)

// XYChart is the content of `@visx/xychart`
import { XYChart } from '@visx/visx';

() => (
  <XYChart.XYChart>
    <XYChart.LineSeries {...} />
  </XYChart.XYChart>
)

// or destructure like
import { XYChart as XYChartPackage } from '@visx/visx';
const { XYChart, LineSeries, ... } = XYChartPackage;

() => (
  <XYChart>
    <LineSeries {...} />
  </XYChart>
)

@kristw @hshoff
cc @valtism

[coveralls]

Pull Request Test Coverage Report for Build 532451544

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 61.749%

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Totals
Change from base Build 516508911:	0.0%
Covered Lines:	1786
Relevant Lines:	2753

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💛 - Coveralls