fix(annotation/Label): fix doc types and padding

[williaster]

📝 Documentation

🐛 Bug Fix

This fixes a couple of small issues with Label from @visx/annotation:

• the prop types for Label were not correctly generated in docs/annotation because the exported component name did not match the file name (this is a limitation of the TS doc type generation, should turn on the lint rule for this)
• Label's props.backgroundPadding can be of shape number | { top?: number, right: ..., bottom: ..., left: ... }. the logic for the object shape was currently broken / this fixes it.
• there was no default for anchorLineStroke, so if you set showAnchorLine={true} it wasn't visible by default. should have a default so users aren't confused!
• Previously the /annotation demo didn't update the annotation position on screen size changes. This results in a broken example in codesandbox when it initially renders with 0 width/height or if the dimensions changed. Added a fix.

Here's the verified doc fix:

Before/after in code sandbox:

@hshoff

[coveralls]

Pull Request Test Coverage Report for Build 467004564

• 0 of 1 (0.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 61.203%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
packages/visx-annotation/src/components/Label.tsx	0	1	0.0%

Totals
Change from base Build 462020185:	0.03%
Covered Lines:	1728
Relevant Lines:	2690

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💛 - Coveralls