typescript(vx-group): re-write package in TypeScript

[williaster]

🚀 Enhancements

This PR builds off of #484, introducing TypeScript config and converting the @vx/group package (which is quite simple) as a proof of concept. The plan is to convert packages one by one from here 📈

Notes

• @types/* related to testing were added to the root devDeps
• package-specific types should be added as deps in the relevant packages
• @airbnb/config-babel adds babel-plugin-typescript-to-proptypes when used with the typescript driver, which generates PropTypes for react component types/interfaces and requires us to keep the prop-types dep though it's no longer referenced in source code. Example compiled .js file below:

Closes #510

@hshoff @milesj @schillerk @techniq

Fixes https://github.com/hshoff/vx/projects/2#card-26383081

[williaster]

whoops, looks like I failed to update the branch this should be merged into to master, after chris--nimbus was merged. should I open another PR for the same into master?