Moment: [ES Modules] Missing js extensions in import statements

Adding the .js file extension to all import under src/ could easily lead to a chain of up to 105-228 HTTP requests (depending on locales)

A more appropriate result might be achieved by adding esnext and module properties to package.json. From a series of articles by Axel Rauschmayer it seems like you'd want something like:

  "esnext": "src/moment.js",
  "module": "./moment.esm.js",

(where ./moment.esm.js would be a new build target, following ESM instead of CJS module format)

+1 for a new esm build target. For reference, Ractive.js provides an .mjs dist file

I'll take a PR on documentation about this in https://github.com/moment/momentjs.com

Btw, even with a good number of HTTP requests, it can be convenient to have browser builds for use in demos--where the demos aren't forced to pollute their HTML with global script dependencies (I know it'd just be one script tag if there is an IIFE or UMD build, but a script tag is still out of place from the JS that uses it and cleaner to have a module distribution).

@brettz9 I'll look at PRs that don't break any existing users.