Add unpkg field to package.json

[tmcw]

Hello! Zdog is amazing 😍

This PR adds an unpkg field, as recommended by unpkg to correctly resolve the UMD build of the module. This means that, first, folks using d3-require automatically get the right version. And also, people can use https://unpkg.com/zdog@1 to automatically get the right file for a given semver version range, even if you move around the dist file (so if you want to move dist, it's less likely to automatically be a major change. Plus, it'll make Zdog a bit easier to require on Observable!

[desandro]

Good call! Thank you for this contribution. And welcome to open source! This feature will ship in the next release. Email me yo@metafizzy.co if you like stickers!