new(xychart): add annotations

[williaster]

🚀 Enhancements

This PR integrates the new @visx/annotations into @visx/xychart and updates the demo. Most of the work is wrappers around the annotation components to provide theme + dimensions from XYChart's context, and I also added an AnimatedAnnotation variant so that they can animate upon data changes.

A quick summary of components added for ease of review

• private/BaseAnnotation powers Annotation and AnimatedAnnotation (mirrors how AnimatedSeries work)
• Annotation
• AnimatedAnnotation

Animation

Editable

Theme support

Horizontal line annotations

Noting that you can render an annotation per data point, so this PR fixes #903

Something like this

{data.map((d, i) => (
  <AnimatedAnnotation key={i} dataKey="New York" datum={d} dx={0} dy={-10}>
    <AnnotationLabel
      subtitle={`${d['New York']}°F`}
      horizontalAnchor="middle"
      verticalAnchor="middle"
      showBackground={false}
      showAnchorLine={false}
      width={50}
    />
  </AnimatedAnnotation>
))}

💥 Breaking Changes

This updates the shape of the xychart theme, but since it's not released it's not technically breaking in a semantic version sense.

@kristw @hshoff

[williaster]

this just makes it so we render 3 lines when we don't render a bar series, ditto on area below

[williaster]

need to make this optional since we allow you to override x/yAccessors

[williaster]

in this file, just added the annotation state + re-ordered several controls to group them more logically (lots of controls were added over time so it's a little overwhelming)

[kristw]

lgtm

[coveralls]

Pull Request Test Coverage Report for Build 383875944

• 9 of 45 (20.0%) changed or added relevant lines in 10 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-1.06%) to 59.864%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
packages/visx-xychart/src/components/annotation/Annotation.tsx	0	1	0.0%
packages/visx-xychart/src/components/annotation/AnnotationCircleSubject.tsx	0	2	0.0%
packages/visx-xychart/src/components/annotation/AnnotationConnector.tsx	0	2	0.0%
packages/visx-xychart/src/components/annotation/AnnotationLineSubject.tsx	0	2	0.0%
packages/visx-xychart/src/components/annotation/AnnotationLabel.tsx	1	5	20.0%
packages/visx-xychart/src/components/annotation/AnimatedAnnotation.tsx	0	9	0.0%
packages/visx-xychart/src/components/annotation/private/BaseAnnotation.tsx	1	17	5.88%

Totals
Change from base Build 382529313:	-1.06%
Covered Lines:	1634
Relevant Lines:	2571

---

💛 - Coveralls

[williaster]

will add some tests in another PR to get coverage pack up