Add ESM bundle

[opening issue as requested]

Included ESM entry point `dist/lib-es6/kosher-zmanim.ts` is not bundled.
Would be useful for use from browser, playgrounds, Deno, Bun, etc.

(Although in many use cases unbundled ESM is actually fine, I seemed to get an error about the export `Big` not being found. esm.sh's bundle worked fine, though)
