new(drag): allow x,y,dx,dy overrides in useDrag + Drag

[williaster]

🚀 Enhancements

This PR builds on #902 and adds the ability to

• set default x/y/dx/dy for useDrag + <Drag />
• update x/y/dx/dy passed to useDrag + <Drag /> such that the internal state of the hook + component update to those values

I added tests for this + updated the /drag-i demo to demonstrate how to set an initial x/y (and verified that it works as before).

Closes #905, and I also need this for some @visx/annotations work that is in progress (validated it for that use case as well).

@hshoff @kristw cc @m0t0r

[coveralls]

Pull Request Test Coverage Report for Build 348135996

• 5 of 7 (71.43%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.5%) to 60.611%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
packages/visx-drag/src/Drag.tsx	0	1	0.0%
packages/visx-drag/src/useDrag.ts	5	6	83.33%

Totals
Change from base Build 348110012:	0.5%
Covered Lines:	1567
Relevant Lines:	2457

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💛 - Coveralls