switch to npm, cleanup rollup, remove umd

[ianstormtaylor]

This cleans up the build process:

• Rely on TypeScript to do all the transforming instead of Babel.
• Get rid of extra ./umd bundles, since Unpkg makes using the CJS one easy.
• Switch to npm from Yarn, to simplify things.
• Remove unused files, ignores, deps, etc.

Another nice thing would be to switch to Vitest from Mocha, allowing us to fully remove all the Babel-related junk. But that's for another time.