Feature/esm as default transformer

[sebastianrothe]

FYI: this is based on #100. So if #100 gets merged first, the changes here a much smaller.

This change makes the ESM async transformer the default. It also adds an error, if it used in the wrong context (e.g. async in CJS). This should fix most of the problems (#97 #91 #86 #72), because it will let Jest use the correct transformer.

It adds documentation with a manual override for all users, who still have to use CJS.

[sebastianrothe]

Interesting read about dual-package libs https://gist.github.com/sindresorhus/a39789f98801d908bbc7ff3ecc99d99c?permalink_comment_id=3850849#gistcomment-3850849

[benmccann]

Sharing something I learned, which is the reason you had to write the PR this way, is that jest always prefers the CJS version via its requireOrImportModule method: https://github.com/jestjs/jest/blob/2499cf2a3a75824fd0c9b454baa95fc75929f086/packages/jest-transform/src/ScriptTransformer.ts#L24

With Svelte 4, CJS won't work, so I think this PR is a good idea. Svelte 4 ships a CJS version of the compiler for use in eslint, but it doesn't ship a CJS version of the runtime, so I don't think it'd be possible to run tests in CJS mode. It might make sense to avoid removing CJS support just yet so that people can still use CJS with Svelte 3, so I think I like just making ESM the default as you've done here and leaving CJS support in place while we see how much trouble or success people have in migrating. If it turns out we really screwed things up by removing CJS support too early in Svelte, it would make it slightly easier to add back to already have CJS support here - though I hope that wouldn't be necessary.

Somewhat surprising to me, it turns out that Jest is still used much more than Vitest or Playwright are with Svelte eventhough the latter two are the ones we offer to setup people up with when setting up new SvelteKit projects. Hopefully Svelte and svelte-jester dropping CJS won't break too many people too badly, but I do know jest's ESM support is immature (not entirely their fault as they depend on node who depend on v8).

[benmccann]

we need to omit the format option with Svelte 4 or else users will get errors printed to the console. I think we can do something like import { VERSION } from 'svelte'; to check what version users are on. @sveltejs/vite-plugin-svelte does this

[benmccann]

I think it's easier to start fresh with this PR rather than address all the merge conflicts, so I'm going to go ahead and close it. I sent #133 with the core changes from this PR, so please give that one a look if you've got a minute!