Add XYChart to @visx/visx

[ShaneHudson]

This PR relates to #974 to ensure XYChart is in the package that includes all other packages.

💥 Breaking Changes

• none

🚀 Enhancements

• Adds XYChart to the @visx/visx package.

📝 Documentation

• none

🐛 Bug Fix

• none

🏠 Internal

• none

[ShaneHudson]

The build failed, it says to regenerate types but not sure how.

[williaster]

@ShaneHudson thanks for the PR! ❤️ sorry for the review delay (holidays 🎁 ).

I think this is failing because some of the named exports from XYChart conflict with exports from different packages 😱 this is (surprisingly) the first time for name clashes, so may need to rethink the approach. I think probably the best thing is to move toward name-spacing the exports into separate files to avoid this. Need to be cognizant of breaking changes tho so I would propose:

1. (this PR, no breaking changes): create a new src/xychart.ts file which has the export * from '@visx/xychart'; line and avoids name conflicts.
2. (follow up PR, consistency with no breaking changes): create a new src/xxx.ts for each package, e.g., src/shape.ts would export * from '@visx/shape';. the src/index.ts is unchanged.
3. (follow up PR, for breaking 2.0.0 release): remove src/index.ts and remove package.json main + module entries which reference this index.

I'll probably create an issue to track the multiple parts. If you want to combine 1 and 2 into this PR feel free else I'm happy to take a stab 🔜 . There's no planned 2.0.0 yet, but I'll start a project with known todo items for it.