Migrating to ESM only

Hello

today, this package has multiple copies/entries in it:

• dist/diff.js - a UMD bundle
  • Only exposed under package.json#browser and package.json#unpkg keys
  • Suggests this is the browser bundle
• dist/diff.min.js - the same UMD bundle minified
• libjs/ - a CommonJS entry point
  • Includes types
  • Includes sourcemaps
• libesm/ - an ES module entry point
  • Includes types
  • Includes sourcemaps

Times have changed a bunch since this package was created. We're now in a situation where:

• All browsers support ES modules
• All runtimes (e.g. Node, Bun, etc.) support ES modules
• CommonJS projects in Node can require(esm) an ES module
• Most web projects today are using a bundler

So it seems there's opportunity here for a new major one day which dumbs this down to:

• dist/ - an ES module entry point
  • Includes types
  • Includes sourcemaps

would love to hear your thoughts on this @kpdecker

an extra win from this could also be to stop shipping sourcemaps (which account for a large % of the tarball size we pull from npm). The code is already readable, unminified, and has type definitions. So we're not gaining much from shipping them.