Port to ESM

[edemaine]

As discussed in #34, this PR ports the project to ESM. Tests work locally; we'll see if I fixed CI correctly.

I tested in the browser with the following code (to get a global variable):

<script type="module">
  import stringify from "./index.js";
  window.stringify = stringify;
</script>

I personally find #34 slightly more useful because it allows me to run everything in file: URLs; when testing, ESM import only works if I run a local server. But this code is definitely cleaner.

Some notes:

• eslint doesn't currently support ESM configuration, so we have to use .eslintrc.cjs
• Jest requires --experimental-vm-modules so that it can manipulate ESMs.
• Jest is broken by Node 16.6.0; see Error after upgrading to Node 16.6.0 when using ES modules jestjs/jest#11708. It should be fixed in 16.6.1 tomorrow.

[lydell]

Hi! Just wanted to let you know I haven’t forgotten about this. I want to come back to this later this year.

[lydell]

New year, still not done. It’s still on my list though.

[lydell]

I finally made the decision to release as ESM only! I didn’t merge your PR in the end, since I decided to get rid of Jest. But thanks anyway for the PR! Version 4.0.0 is now available on npm.