Investigate potential `--esm` make flag

[supermario]

Basically the stuff that https://www.npmjs.com/package/elm-esm does but native.

Elm's current IIFE (immediately invoked function expression) setup is here:

compiler/compiler/src/Generate/JavaScript.hs

Line 50 in 9612b17

"(function(scope){\n'use strict';"

Stuff to look into

• Can a generateEsm function be written right next to there that does what we need?
  • If not, what other changes does ESM output require?