Remove dist/ folder

[donmccurdy]

Including the dist/ folder in source control is a regular source of confusion for new contributors, because the files are modified when building or testing changes, but shouldn't be included in PRs. These files cannot be reviewed easily, and are meant to be rebuilt by an author when publishing a new release. To avoid this confusion, the dist/ folder should be added to .gitignore and explicitly added to the published files in package.json. Because it's published to NPM, it will still be available from CDNs like Unpkg.