feat(esm): Adding experimental support for ESM apps (1/n)

[Tobbe]

With #80 in such a great place ✅ I had to decide on how to actually merge it into the code base

I had two options in my mind

1. Make feat(create-cedar-app): Create ESM apps #80 a hard breaking change. Remove all CJS support, including jest etc and go ESM only
2. Try to support both CJS and ESM projects

I was going to do option 2 first, but then when I saw how much #80 grew over time (it was at over 400 changed files at one point!) I started leaning more and more towards 1. So I updated all templates, test fixtures, generators etc to only use ESM syntax.

If I did option 1 I'd release ESM-only CedarJS as v1.0.0, and that version could then also include a few other breaking changes I have lined up.

But as I started thinking more about it I'm back at option 2.
If I add ESM support in a backwards compatible way I can release it as minor versions. That way I can potentially get more people to try it out earlier/easier. And I can release smaller major versions with other breaking changes alongside working on full ESM support.

So this PR is the first step of adding ESM-only code in a way that's backwards compatible. I start by adding ESM test fixtures and templates. There's currently no way to set up a new Cedar app using those templates, and the test fixtures aren't used yet. But this lets me merge a big chunk of the ESM changes in #80 without any disruption to existing apps or disruption to my ability to release new patches and minor versions of CedarJS

For posterity, this is where #80 was at when I created this PR:

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